Full opinion text
COFFEY, Circuit Judge. This is the consolidated appeal of six defendants convicted of various federal narcotics crimes. We affirm the convictions of each of the defendants and the sentence of Roberto Ramirez. I. PROCEDURAL BACKGROUND On January 21, 1988, a grand jury returned a three-count superseding indictment charging violations of 21 U.S.C. §§ 841(a)(1) and 846. Count I of the indictment charged Roberto Gonzalez, Roberto Ramirez, Rafael Izquierdo, Angel M. Ame-jeiras, Vicente Chao, and Luis Gonzalez, together with Jose Alexander Peña, Ulises Ortega, Cesar Tobon, Manuel Angel Telle-chea, Alvaro Quintero and Alcides Cruz with conspiracy to possess with intent to distribute and to distribute cocaine in violation of 21 U.S.C. §§ 841(a)(1) and 846. Count II charged that on July 29, 1987, Vicente Chao and Ulises Ortega knowingly and intentionally possessed with intent to distribute approximately 2,265 kilograms of a mixture containing cocaine, a “Schedule II narcotic drug controlled substance,” in violation of 21 U.S.C. § 841(a)(1) and that Roberto Gonzalez, Luis Gonzalez, Ramirez, Amejeiras, Izquierdo, Tellechea, Quintero and Cruz aided and abetted in the commission of this offense. Count III alleged that, also on July 29, 1987, Jose Alexander Peña and Cesar Tobon knowingly and intentionally possessed with intent to distribute approximately 2,248 kilograms of cocaine in violation of 21 U.S.C. § 841(a)(1) and that Roberto Gonzalez, Luis Gonzalez, Ramirez, Amejeiras, Izquierdo, Chao, Telle-chea, Quintero and Cruz, aided and abetted in the commission of the offense. The appellants, together with Alcides Cruz and Ulises Ortega, proceeded to trial on April 4, 1988. The jury returned a verdict finding the appellants and Alcides Cruz guilty on all counts but finding Ulises Ortega not guilty. The defendants, sentenced to various terms of confinement, appeal their convictions. II. FACTUAL BACKGROUND This case involves a cocaine importation and distribution conspiracy which imported cocaine into Miami, Florida through Honduras during the years 1986 and 1987. The cocaine was concealed in shipments of plantains. From Miami the drugs were distributed to the cities of New York and Chicago. Testimony concerning the conspiracy’s operation was received from the defendant Cesar Tobon, a member of the conspiracy who participated in the cocaine distribution operation. Testimony was also provided by unindicted, co-conspirators Jesus Ortiz and Julio Ortiz, father and son, who participated in the conspiracy’s cocaine importation operations and testified under grants of immunity. Daniel Petroski, a truck driver, who was unwittingly hired to transport the conspiracy’s July 1987 drug shipment also provided testimony. Jesus Ortiz testified that two shipments of cocaine were made, in August and October 1986 respectively, from Honduras to Miami by a group consisting of Jesus Ortiz, Alcides Cruz, Manuel Angel Tellechea, and Roberto Ramirez. The cocaine was brought to Honduras from Columbia via airplane. In Honduras, the cocaine was hidden in plantain shipment containers placed aboard ships destined for Miami. As a result of one of the airplane shipments of cocaine from Columbia to Honduras being intercepted by the Honduran military in October of 1986, the cocaine importation enterprise was temporarily suspended. Julio Ortiz testified concerning the conspirators’ efforts to resume the drug importation and distribution activity after the October 1986 incident and described a February 1987 meeting held between Cruz, Tel-lechea, Angel Amejeiras and himself, where Cruz outlined his plans for the future operations of the conspiracy. During this meeting Cruz explained that Julio would be in charge of the import company receiving the cocaine in Miami, Roberto Ramirez would supply the drugs, and that Cruz and Tellechea would be responsible for the transportation and handling of the shipments in Central America. In late April or early May of 1987, Julio Ortiz commenced the operation of the Miami-based import business, International World Enterprises, to receive the conspiracy’s cocaine shipments from Honduras. A shipment said to contain 1,148 kilograms of cocaine concealed in plantains arrived in Florida on a freighter on June 4, 1987. Shortly thereafter, as a result of a disagreement over money, Julio Ortiz left his position as head of International World Enterprises. Luis Gonzalez became the manager of a new company known as International Palm Products, that replaced International World Enterprises as the outlet for the receiving of cocaine imported into Miami from Honduras. Cesar Tobon described the New York cocaine distribution operation, responsible for the transportation of the cocaine imported through Miami for the New York area. Tobon spoke of the numerous trips that he and other members of the conspiracy made in June and July 1987 between New York and Florida and within the New York City area. He explained further that Roberto Gonzalez was the recipient of many of these cocaine deliveries. In mid-July 1987 the conspiracy shipped 2,265 kilograms of cocaine, concealed in a load of plantains, from Honduras to Miami. Unfortunately for the conspirators, the large cocaine shipment was intercepted in Miami on July 23, 1987, by a special team of Customs agents. At this time federal authorities set up a “controlled delivery” of the cocaine. During the time the cocaine was being intercepted, Tellechea and Amejeiras were at a truck stop in Miami where they met with one Daniel Petroski to hire him to deliver what Petroski believed to be a load of plantains to Las Villas Enterprises in the Bronx, New York. Petroski picked up the load of plantains which, unbeknownst to him contained cocaine, at Miami Cold Storage and left for New York. After departing from Miami Cold Storage, he and his shipment were detained by federal agents in the Miami area. After questioning, he agreed to cooperate in the drug investigation. Petroski testified that when he met Luis Gonzalez at the Las Villas Enterprises building in the Bronx on July 28, 1987, Gonzalez stated that a mistake had been made as to the destination of the cargo and that it should be delivered to Vicente Chao in Chicago. Petroski further testified that at this time Luis Gonzalez entered the trailer and inspected the cargo with a flashlight. On the morning of July 29, Rafael Izquierdo, who had traveled from Miami to Chicago with the conspiracy’s cocaine supplier (Roberto Ramirez) went to the Celi-Mar fish, vegetable and fruit dealership in Chicago where Chao was employed. Izquierdo later departed with Celi-Mar’s owner, Matteo Hernandez, to visit area stores in a supposed attempt to generate fish sales. Hernandez testified that this was the first time that he had ever made the rounds of area businesses with Izquier-do. Izquierdo apparently accompanied Hernandez to ensure that Hernandez would not return while the cocaine was being transferred. Testimony from Petroski and federal agents established that Petroski arrived at the Celi-Mar warehouse in Chicago, Illinois, shortly after noon on July 29, 1987. This testimony, together with a videotape of the cocaine delivery to the Celi-Mar warehouse introduced at trial, revealed that Chao and the defendant Ulises Ortega commenced the unloading of Petroski’s truck. Later, drug couriers Jose Alexander Peña and Cesar Tobon arrived at the Celi-Mar warehouse and conferred with Chao. Chao explained to Pena and Tobon that they could locate the boxes containing the cocaine by placing their fingers in the containers’ air holes and feeling the plastic wrap rather than plantains. During the loading Tobon organized the cargo in the truck while Peña sought to locate a missing box of cocaine. After the cocaine-laden truck departed from the warehouse, law enforcement officers intercepted and confiscated the truck and arrested Peña and Tobon. Chao was also arrested the same day. Following the arrests of Chao, Peña and Tobon, law enforcement officers conducted a search of the Celi-Mar warehouse. In the aftermath of the arrests other conspirators who had been in the Chicago area awaiting the delivery departed, including Luis Gonzalez and Telleehea who took circuitous routes home in an attempt to confuse pursuers. Steven Bissegger, Special Agent for the DEA, testified that around noon on August 4, 1987, he “received a phone call ... from an individual who identified himself as Gary Sternberg” and “said he was an attorney who represented Mr. Matteo Hernandez.” Bissegger further testified that after he received the phone call, he “proceeded to Celi-Mar” with another DEA agent. When the agents arrived at the cold storage area of the Celi-Mar warehouse, Bissegger stated he met “an individual who identified himself as Mr. Sternberg” who “pointed out a wooden pallet containing some boxes,” and told him “that a box had been found on that pallet that he thought we would be interested in.” Bissegger stated that Sternberg pointed to a particular box. Bissegger went on to testify that when the box was opened he viewed “17 individually wrapped packages.” The contents of the box were submitted to the DEA laboratory where it was determined that the box “contained 17 ... separate packages, weighing a total of 17.-027 kilograms that contained powder that was tested and determined to be 93 percent pure cocaine.” III. ISSUES PRESENTED (1) Was it proper for Ramirez to receive multiple punishments for convictions on two separate counts of possessing cocaine with intent to distribute; (2) Did the trial court properly refuse to sever the trials of Ramirez and Izquierdo from those of their co-defendants; (3) As to all the defendants, did the trial court err in admitting and allowing the jury to view 2,248 kilograms of cocaine in the courtroom; (4) With respect to Chao, did the district court abuse its discretion in admitting expert testimony concerning the unusual length of narcotics-oriented telephone calls; (5) Was Chao's Fifth Amendment privilege against self-incrimination violated when the trial court received in evidence Chao’s affidavit of in-digency; (6) Did the prosecutor’s reference in closing argument to facts not in evidence pertaining to Chao’s alleged theft of a missing box of cocaine deny Chao a fair trial; (7) Did the trial court deny Luis Gonzalez a fair trial in giving the jury a “conscious avoidance” instruction pertinent to its consideration of the evidence against him; (8) Was there sufficient evidence to support Chao’s, Izquierdo’s, Amejeiras’ and Luis Gonzalez’ convictions of (a) conspiracy to possess cocaine with intent to distribute, (b) possession with intent to distribute and (c) aiding and abetting possession of cocaine with intent to distribute; (9) Was the trial court’s refusal to give Roberto Gonzalez’ proffered instruction requiring “membership” in the conspiracy to establish vicarious liability on the two possession with intent to distribute counts proper together with the related question of whether sufficient evidence supported Roberto Gonzalez’ convictions on the substantive counts; and (10) Did the government’s failure to disclose information concerning alleged prior drug activity of government witness Cesar Tobon require that a new trial be granted to Izquierdo, Chao, and Roberto Gonzalez. IV. MULTIPLICITY OF INDICTMENT Defendant Roberto Ramirez argues that the indictment was multiplicitous because the two possession with intent to distribute counts in the indictment charged the commission of merely one offense. In United States v. Briscoe, 896 F.2d 1476, 1522 (7th Cir.1990), we observed that: “Essentially a claim of multiplicity alleges that separate counts in an indictment charge a single offense. ‘As such the indictment exposes a defendant to the threat of receiving multiple punishment for the same offense.’ ” (Citation omitted) (quoting United States v. Podell, 869 F.2d 328, 330 (7th Cir.1989)). In United States v. Marquardt, 786 F.2d 771, 778 (7th Cir.1986), we set forth the standard utilized in evaluating whether counts are multiplicitous: “The traditional test of multiplicity ‘determines whether each count “requires proof of a fact which the other does not.” ’ United States v. Kennedy, 726 F.2d 546, 547-48 (9th Cir.), cert. denied, [469 U.S. 965], 105 S.Ct. 365, 83 L.Ed.2d 301 (1984) (quoting United States v. Glanton, 707 F.2d 1238 (11th Cir.1983)). ‘If one element is required to prove the offense in one count which is not required to prove the offense in the second count, there is no multiplicity.’ United States v. Briscoe, 742 F.2d 842, 845 (5th Cir.1984).” Ramirez was convicted of the two counts of possession with intent to distribute cocaine based upon the vicarious liability theory of Pinkerton v. United States, 328 U.S. 640, 66 S.Ct. 1180, 90 L.Ed. 1489 (1946). In order for Ramirez to be liable for Chao or Ortega’s possession of cocaine with intent to distribute under Pinkerton, the government was required to demonstrate that Chao or Ortega possessed cocaine with intent to distribute in furtherance of or as a natural consequence of the conspiracy and that Ramirez was a co-conspirator with either Chao or Ortega. Conviction on the second count of possession with intent to distribute required proof that Peña or Tobon (rather than Chao or Ortega) had possessed cocaine with intent to distribute in furtherance of or as a natural consequence of the conspiracy and that Ramirez was a co-conspirator with Peña or Tobon. The record establishes that these two separate crimes of possession of cocaine with the intent to distribute in conjunction with the conspiracy occurred and that Chao (the jury absolved Ortega) possessed the cocaine with the intent to distribute it at the time he and Ortega unloaded it at the Celi-Mar warehouse, and Ramirez is vicariously liable for Chao’s crime under Pinkerton in Count II. Likewise, under Count III Ramirez is vicariously liable for Peña’s and Tobon’s possession of the cocaine with the intent to distribute it, which occurred when they conveyed the cocaine from the warehouse in a truck. Since the government was required to prove the element of Peña’s and Tobon’s possession of cocaine with the intent to distribute under Count III that was immaterial to Count II, it was proper that the two counts be individually charged and that Ramirez receive separate punishment for each count. V. SEVERANCE Ramirez and Izquierdo both sought to separate the trials of their cases from the joint trial of the co-defendants. Ramirez argued that severance was necessary because two of his co-defendants, Vicente Chao and Rafael Izquierdo, might have provided testimony favorable to Ramirez in a separate trial. Izquierdo argued that his case should be severed because there was a great disparity between the degree of evidence against him as compared with the stronger case against the co-defendants, Roberto Gonzalez, Luis Gonzalez, Roberto Ramirez, Angel Amejeiras, Alcides Cruz and Vicente Chao. In United States v. Moya-Gomez, 860 F.2d 706, 754 (7th Cir.1988), cert. denied, 492 U.S. 908, 109 S.Ct. 3221, 106 L.Ed.2d 571 (1989), we summarized the standard utilized in reviewing a district court’s severance decision: “Rule 14 permits the trial court in the exercise of its discretion to grant separate trials when the interests of justice so require. A district court’s ruling on a Rule 14 severance motion will be overturned only upon a showing of abuse of discretion. Because the balancing of the cost of conducting separate trials and the possible prejudice inherent in a single trial is best conducted by the trial court, the defendant bears an extremely difficult burden of showing on appeal that the district court abused its discretion. In order to appeal successfully the denial of a severance motion, a defendant must establish actual prejudice resulting from the denial. Actual prejudice means that the defendant could not have a fair trial without severance, ‘ “not merely that a separate trial would afford him a better chance of acquittal.” ’ [United States v. Peters, 791 F.2d 1270, 1301 (7th Cir.), cert. denied, 479 U.S. 847, 107 S.Ct. 168, 93 L.Ed.2d 106 (1986)] (quoting United States v. Papia, 560 F.2d 827, 836 (7th Cir.1977)) ....” (Footnotes and citations omitted). We have also noted the strong public interest in joint trials in conspiracy cases: “ ‘In considering a motion for severance, the trial judge should give deference to the “strong public interest in having persons jointly indicted tried together, particularly where, as here, a conspiracy is charged and may be proved by evidence that arises out of the same act or series of acts.” ’ United States v. Percival, 756 F.2d 600, 610 (7th Cir.1985) (quoting Papia, 560 F.2d at 836-37)....” Briscoe, 896 F.2d at 1516. A. Ramirez’ Severance Claim — Presentation of Testimony of a Co-Defendant in a Severed Trial Ramirez alleges that his trial should have been severed from his co-defendants’ in order that he might be permitted to present exculpatory testimony of co-defendants Vicente Chao and Rafael Izquierdo. In support of his pretrial motion for severance, Ramirez supplied an affidavit of Vicente Chao stating that Chao would testify in a separate trial of Roberto Ramirez that Ramirez had traveled to Chicago during the time period of the conspiracy in furtherance of legitimate business dealings. Ramirez also furnished a pretrial affidavit of Rafael Izquierdo which stated that Izquierdo would testify that Ramirez traveled to Chicago on legitimate business during the conspiracy’s time frame and that one of the purposes of Ramirez’ trip to Chicago was to discuss financing of a transaction that would permit Izquierdo to purchase Ramirez’ business, Agromar Enterprises. It is interesting to note that neither of the affidavits contained statements that excluded the possibility that Izquierdo also traveled to Chicago on drug-related business on other occasions during the conspiracy nor specifically discussed, much less detailed with any specificity, Ramirez’ July 1987 trip to Chicago. The government relied upon Ramirez’ July 1987 trip to Chicago in establishing his guilt. In United States v. Studley, 892 F.2d 518, 525 (7th Cir.1989), we held that: “When a defendant seeks a severance to avail himself of allegedly exculpatory testimony from a co-defendant, a trial court should consider three factors: ‘(1) whether the co-defendant’s testimony would be exculpatory; (2) whether the co-defendant would in fact testify; and (3) whether the testimony would bear on defendant’s case.’ United States v. Melton, 689 F.2d 679, 686 (7th Cir.1982) (citations omitted).” As detailed above, the problem with Ramirez’ severance claim is that the content of the statements presented in the affidavits of Chao and Izquierdo would not exculpate Ramirez from allegations that Ramirez took a trip to Chicago for drug-related purposes. They merely state that Ramirez took a legitimate trip to Chicago. Thus, the district court did not abuse its discretion in denying Ramirez' severance motion. B. Izquierdo’s Motion for Severance — ‘ ‘Spillover” Effect of Evidence Izquierdo alleges that the trial court should have severed his trial because the evidence presented against his co-defendants was much stronger than the evidence introduced against Izquierdo. In United States v. Diaz, 876 F.2d 1344, 1357 (7th Cir.1989), we summarized the legal principles applicable to a severance claim of this nature: “ ‘Denial of a motion for severance may be an abuse of discretion if there is a great disparity of evidence between the moving defendant and [his] codefend-ants.' Moya-Gomez, 860 F.2d at 765. But, ‘the fact that the evidence against [his] co-defendants might have been proportionally greater than the evidence against [him] is not itself grounds for a severance.’ United States v. Hendrix, 752 F.2d 1226, 1232 (7th Cir.1985). ‘In such situations, the relevant inquiry is whether it is within the jury’s capacity to follow the trial court’s limiting instructions requiring separate consideration for each defendant and the evidence admitted against [him].’ Moya-Gomez, 860 F.2d at 765. ‘Juries, however, are presumed capable of sorting through the evidence and considering the cause of each defendant separately.’ United States v. Williams, 858 F.2d 1218, 1225 (7th Cir.1988).” 876 F.2d at 1357-58. In this case the district court provided the jury with the following instruction: “Although the defendants are being tried jointly, you must give separate consideration to each defendant. In doing so, you must analyze what the evidence in the case shows with respect to each defendant, leaving out of consideration any evidence admitted solely against some other defendant or defendants. Each defendant is entitled to have his case decided on the evidence and the law applicable to him.” The district court further instructed the jury that: “Where two or more persons are charged with the commission of a crime, the guilt of one defendant may be established without proof that each of the defendants performed every act constituting the crime charged. However, you must give separate consideration to each individual defendant and to each separate charge against him. Each defendant is entitled to have his case determined from his own conduct and from the evidence which may be applicable to him.” Izquierdo, despite all the evidence against him, asserts that this proof is less probative than the facts presented against the other defendants. But in previous cases involving allegations of disparate evidence, limiting instructions of the nature enumerated above have been held sufficient to eliminate any prejudice resulting from alleged “spillover” of evidence applicable to other defendants. See United States v. Briscoe, 896 F.2d at 1517-18; United States v. Diaz, 876 F.2d 1344, 1358 (7th Cir.1989). Because the jury was properly instructed and directed to give individual consideration to each and every element of each crime charged against a particular defendant and because “juries ... are presumed capable of sorting through the evidence and considering the cause of each defendant separately,” we are convinced that a jury, following these instructions, was able to give separate consideration to Izquierdo’s case. Izquierdo, thus, did not suffer prejudice as a result of the evidence introduced at the trial. VI. PHYSICAL DISPLAY AND ADMISSION INTO EVIDENCE OF 2,248 KILOGRAMS OF COCAINE SEIZED FROM DEFENDANTS Each of the defendants argues that the trial court acted improperly in admitting and displaying the 2,248 kilograms of cocaine seized from the defendants Peña and Tobon before the jury. The admission and display of this large volume of cocaine was the subject of repeated objections at trial. When the prosecution announced at the opening of the trial that it intended to exhibit this amount of cocaine to the jury, the defendants objected, stating that this would result in a “sideshow” and that it was an “attempt to inflame the jury with the quantity of cocaine_” The defendants went on to state that they would not object to the government’s introduction in evidence of one or two kilograms of cocaine and would stipulate only that this cocaine was part of a shipment of 2,260 kilograms, that it was in fact cocaine and that there was an uninterrupted chain of physical custody. The defendants also suggested that photographs or videotape could adequately portray the quantity of cocaine to the jury. The court, after hearing argument, overruled the objection and allowed the admission of the cocaine and the display of the quantity to the jury for a limited period of time stating that this case involved the transportation of a large volume of cocaine: “Well, I think that in view of the fact that a fair amount of testimony in this case is going to involve moving it [the cocaine] from one place to another and in this, out of that and so forth, that they’re entitled to indicate to the jury what kind of volume they are talking about. But as far as I am concerned, we do it once, they take a look so that the jury has an idea of the kind of volume that we are talking about, because there is going be a lot of testimony to some logistics of it, and then we get it out of here, and we never see it again.” After the cocaine was put on display before the jury, the defendants moved for a mistrial. In denying the motion, the court stated: “I will say, again, it seems to me that it is relevant, and the jury has a right to know what kind of actual volume was involved that somebody had to transport in the center of a shipment of plantains.” The next morning several of the defense attorneys requested the court examine each of the jurors to determine whether the introduction and display of this evidence had a prejudicial effect upon them. The judge denied this request, but stated that he was willing to give the jury a limiting instruction. A number of the defendants requested this instruction and, when the jury returned, it was instructed that: “This is a case where nobody has ever disputed that a large amount of cocaine was imported into the United States by somebody. What the case is all about is whether the government can prove its charges against each of these defendants. We ended up yesterday with all the cocaine being brought in, which, obviously, was a lot of cocaine. It was brought in solely for the purpose of acquainting you with the kind of bulk that was involved and for no other purpose.” The defendants argue that the admission and display of this amount of cocaine to the jury resulted in prejudice outweighing the probative value of the evidence. Rule 403 of the Federal Rules of Evidence provides that: “Although relevant, evidence may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or misleading the jury, or by considerations of undue delay, waste of time, or needless presentation of cumulative evidence.” “It is well established that ‘a trial judge’s assessment of relative probative value and unfair prejudice is generally accorded great deference because of his first-hand exposure to the evidence and his familiarity with the course of the trial proceeding.’ ” Briscoe, 896 F.2d at 1498 (quoting United States v. Liefer, 778 F.2d 1236, 1244 (7th Cir.1985) (citation omitted)). Furthermore, it is recognized that any evidence of a crime carries with it a degree of prejudice. See United States v. Sophie, 900 F.2d 1064, 1076 (7th Cir.1990) (“[A]ll probative evidence is prejudicial to the party against whom it is introduced.”). The amount of cocaine seized (2,248 kilograms) certainly was relevant to establish the magnitude of the conspiracy as well as that the quantity of cocaine could not have been intended for personal use rather than for the purpose of possession with intent to distribute cocaine on a large scale. The jury was given an opportunity to view the drugs in the courtroom for a limited period of time. Furthermore, at the beginning of the following day the jurors were given an instruction that specifically reminded them of the very limited purpose for which the cocaine was introduced (“acquainting [the jury] with the kind of bulk that was involved and for no other purpose”). Thus, we hold that the trial court’s exercise of discretion in admitting into evidence and permitting display of the 2,248 kilograms of cocaine was proper. VII. ADMISSION OF EXPERT TESTIMONY CONCERNING LENGTH OF NARCOTICS-RELATED TELEPHONE CALLS DEA Agent Francis Tucci testified that usually “narcotic-related phone calls or phone calls which deal with narcotic conversations are short. They are against the law, and, therefore, anytime you are committing the crime you make it as short as possible. They are brief.” Chao asserts that Agent Tucci’s statement was “mere speculation” that should not have been admitted into evidence as expert testimony. We have recognized that federal agents trained and experienced in drug-related transactions, crimes and prosecution are qualified to give expert testimony concerning the practices of those engaged in this type of criminal activity. In United States v. Rollins, 862 F.2d 1282, 1292 (7th Cir. 1988), we permitted a federal agent to provide expert testimony concerning the meaning of narcotics code words and phrases, observing: “In our view, the district court properly concluded that the meaning of narcotics code words and phrases is not within the common understanding of most jurors. We therefore agree with a number of courts that have addressed this question, that narcotics code words and the operations of drug dealers are generally an appropriate subject for expert testimony. See United States v. Ginsberg, 758 F.2d 823, 830 (2d Cir. 1985) (‘The operations of narcotics dealers are a proper subject for expert testimony under Rule 702.’); United States v. Ramirez, 796 F.2d 212, 216 (7th Cir.1986) (FBI agent properly qualified as an expert to testify to the meaning of code words used during drug transaction); United States v. Martino, 664 F.2d 860, 864 n. 3 (2d Cir. 1981), cert. denied, 458 U.S. 1110, 102 S.Ct. 3493, 73 L.Ed.2d 1373 (1982) (admission of expert testimony regarding the meaning of certain code words used during drug transaction was appropriate). The district court did not abuse its discretion in permitting Agent Wright to give his expert testimony on the meaning of the code words that Wells and Slaughter used to set-up their drug transactions.” See also Briscoe, 896 F.2d at 1497 (“It is well settled that the meaning of narcotics code words are an appropriate subject for expert testimony”). We stated in United States v. Vega, 860 F.2d 779, 795 (7th Cir. 1988): “As we observed in United States v. Zanin, 831 F.2d 740, 744 (7th Cir.1987): ‘Conversations regarding drug transactions are rarely clear. A fact-finder must always draw inferences from veiled illusions and code words.’ In this case the jury was confronted with conversations which contained ‘code words’ that when considered in isolation, might seem unclear, veiled and almost nonsensical, but when analyzed properly, in the context of the totality of the evidence, can be clearly be seen to be ‘code words’ for drugs. See generally United States v. Abascal, 564 F.2d 821, 827 (2nd Cir.1977) (‘The conversing conspirators frequently discussed non-narcotic related matters at the beginning of conversations, and often resorted to jargon and code words, a frequent practice in narcotics dealings’); United States v. Chavez, 533 F.2d 491, 494 (9th Cir.1976) (‘Jargon and code words are commonly used by those dealing in elicit drugs and were employed here’) (citation omitted). It is true that, advisedly, no explicit mention was ever made of cocaine or other drugs in any of Vega’s conversations with the Zambra-nas. However, a case was made, which was more than strong enough to convince the jury, the trier of fact, that Vega used terms like ‘chickens,’ ‘roosters,’ and ‘it’ as code words for drugs. Not only are code words always used by drug conspirators when they realize as they do in today’s drug culture, that their telephone conversations are frequently intercepted, such terms were obviously used by the conspirators in this case.” Furthermore, we have permitted the admission of expert testimony concerning the operation and use of beepers in drug trafficking. See United States v. Solis, 923 F.2d 548, 550-51 (7th Cir. 1991) (“The government was entitled to demonstrate through the use of expert testimony that someone traveling with two kilograms of cocaine under the conditions we have described would find access to beepers a useful means of effectuating the transportation and eventual distribution of her deadly cargo”). In the same manner that a well trained investigator may properly enlighten a lay jury concerning drug conspirators’ actions including the use of “code words,” he can also inform them of the conspirators’ other practices concerning telephone conversations. The investigator and the expert witness both serve as a link to the drug culture in providing the jury with understanding of the intricate patterns and mo-dus operandi of those participating in drug conspiracies. Furthermore, any questions or problems concerning the expert’s opinion and testimony may be thoroughly explored during the cross-examination of the expert witness. Our examination of the record convinces us that Agent Tueci’s testimony, based upon his experience as a federal drug agent, would be helpful to a jury considering the intricate and clandestine activities of a narcotics conspiracy and, thus, the expert opinion testimony was properly admitted in evidence. VIII. ADMISSION OF CHAO’S AFFIDAVIT OF INDIGENCY Following the completion of the defendants’ cases, the government announced that it would introduce Chao’s affidavit of indigency, prepared for the purposes of obtaining appointed counsel, in its rebuttal case against co-defendants Ramirez and Izquierdo. The reason for this decision was that evidence had been offered in Ramirez’ case that in the summer of 1987 Chao was going to provide financing for Izquierdo to purchase Ramirez’ business, Agromar, and Chao’s indigency would be useful in rebutting this contention. Chao argues that the affidavit of indigency should not have been admitted against his co-defendants because evidence that Chao was indigent would cast doubt upon Chao’s credibility. Specifically, Chao believes that the admission of this evidence against his co-defendants would demonstrate that Chao was “a liar, because he claimed indi-gency when in fact he had a lot of money, or a person who did not want the Government to know about the illegal funds he had acquired.” Although Chao objected to the admission of the evidence at the time of its introduction and in a post-trial motion, he failed to delineate a specific ground for his objection. “To preserve an issue for appellate review, a party must make a proper objection at trial that alerts the court and opposing party to the specific grounds for the objection.... Neither a general objection to the evidence nor a specific objection on other grounds will preserve the issue for review.” United States v. Wynn, 845 F.2d 1439, 1442 (7th Cir. 1988). Thus, Chao “has waived this issue on appeal unless the admission of [the affidavits] was plain error.” Id. In United States v. White, 903 F.2d 457, 466-67 (7th Cir.1990), we explained the “plain error” standard as follows: “As we observed in United States v. Silverstein, 732 F.2d 1338, 1349 (7th Cir. 1984): ‘To be plain, an error must be conspicuous, at least in hindsight....’ As we have also noted: ‘A plain error is an error that is “not only palpably wrong but [is] also likely to cause the outcome of the trial to be mistaken.” United States v. Kehm, 799 F.2d 354, 363 (7th Cir. 1986). “A reversal on the basis of plain error can be justified ‘only when the reviewing court is convinced that it is necessary in order to avert an actual miscarriage of justice.’ ” [United States v. Requarth, 847 F.2d 1249, 1254 (7th Cir. 1988)] (quoting United States v. Silverstein, 732 F.2d 1338, 1349 (7th Cir. 1984), cert. denied, 469 U.S. 1111, 105 S.Ct. 792, 83 L.Ed.2d 785 (1985)).’ United States v. Dietrich, 854 F.2d 1056, 1060 (7th Cir.1988).” “The plain error doctrine allows an appellate court to correct errors that were not objected to at trial if the defendant can demonstrate that he or she probably would have been acquitted but for the erroneously admitted evidence.” Wynn, 845 F.2d at 1443. Chao asserts that his Fifth Amendment privilege against self-incrimination was violated when his affidavit of indigency was offered against Ramirez and Izquierdo but we fail to understand why, for he has failed to demonstrate how admission of this affidavit in the government’s rebuttal case against his co-defendants served to incriminate him. But, this does not end our discussion, for Chao asserts in his appellate brief that the affidavit’s admission was prejudicial because it questioned his credibility. Nonetheless, as discussed in section V-B of the opinion, the court took the precaution to thoroughly instruct the jury concerning its duty to consider the evidence of Chao’s indigency only against Izquierdo and Ramirez, the specific defendants against whom the evidence had been offered. The Supreme Court has observed: “To say that the jury might have been confused amounts to nothing more than an unfounded speculation that the jurors disregarded clear instructions of the court in arriving at their verdict. Our theory of trial relies upon the ability of a jury to follow instructions.” Opper v. United States, 348 U.S. 84, 95, 75 S.Ct. 158, 165, 99 L.Ed. 101 (1954). Because we are convinced the jury was able to follow the specific instructions to consider evidence solely against the parties concerning whom it was introduced, we conclude that the trial court did not commit plain error in receiving the affidavit of indigency in evidence. IX. THE GOVERNMENT’S CLOSING ARGUMENT Chao asserts that his due process right to a fair trial was violated when the prosecutor, during rebuttal closing argument, stated that Chao had stolen the box containing 17 kilograms of cocaine that was missing when the government seized the truck driven by Peña and Tobon. Chao asserts that there was nothing in the record to support this statement and, thus, the government’s statement violated his right to a fair trial. In United States v. Stillwell, 900 F.2d 1104, 1112 (7th Cir.1990), we set forth the law applicable to a defendant’s claim that his rights were violated by an improper government closing argument: “Defendants are entitled to a new trial only if the government’s comments were improper ones that prejudiced the defendants’ rights to a fair trial. It is, of course, improper to suggest that opposing counsel’s objection was not proper or to rely on information not in evidence. It is also improper to inform the jury that there is information not in evidence that supports the government’s case. In determining whether comments prejudiced the defendants’ right to a fair trial, however, the comments must be viewed in the context of the entire trial.” (Citations omitted). In determining whether a defendant’s right to a fair trial was violated as a result of prosecutorial misconduct, we have observed that: “This court’s review of a claim of pros-ecutorial misconduct follows a bifurcated analysis. First, this court examines the challenged remark in isolation to determine whether it was improper. If the court concludes that the remark was improper, the remark is evaluated in light of the entire record to determine whether it deprived the defendant of a fair trial.” United States v. Spivey, 859 F.2d 461, 465 (7th Cir.1988). Initially, we turn to an examination of “the challenged remark in isolation.” Spivey, 859 F.2d at 465. During oral argument the government asserted that the evidence in the record was sufficient to allow the inference that Chao stole the box containing cocaine. The government contends that since Chao had control over the boxes in the warehouse prior to his arrest, he could have removed the box from the shipment before his arrest and might very well have instructed someone to return it subsequent to his arrest. We have been unable to find proof sufficient to support this inference. In United States v. De Geratto, 876 F.2d 576, 586-87 (7th Cir.1989), we determined that a prosecutor presented an argument that when considered, in isolation, was improper where he “went beyond the evidence and stated that the missing record ‘had obviously been suppressed and are sitting in an ashtray somewhere,’ ” and when he accused the defendant “without proof, in the prosecutor’s words, of ‘deep sixing’ the records.” De Geratto, 876 F.2d at 587. Based upon this precedent, reference to Chao’s supposed theft of a box containing 17 kilograms of cocaine was improper in isolation. Under the bifurcated analysis set forth in Spivey, this is only the first step of the two-step process to determine whether there is reversible error. “[T]he remark [must be] evaluated in light of the entire record to determine whether it deprived the defendant of a fair trial.” Spivey, 859 F.2d at 465. In addressing the issue of whether the constitutional right to a fair trial was interfered with as a result of improper prosecutorial comments in United States v. Pirovolos, 844 F.2d 415, 426 (7th Cir.1988), we drew “guidance from the Supreme Court’s ... decision in Darden [v. Wainwright ], 477 U.S. 168, 106 S.Ct. 2464 [91 L.Ed.2d 144 (1986)]. In that case the Court discussed several criteria that may be helpful in our inquiry. They include: (1) the nature and seriousness of the prosecuto-rial misconduct, (2) whether the prosecutor’s statements were invited by impermissible conduct by defense counsel, (3) whether the trial court instructed the jury to disregard the statements, (4) whether the defense was able to counter the improper arguments through rebuttal, and (5) the weight of the evidence against the defendant. Id. at 181-83, 106 S.Ct. at 2472-73.” Just as we refused to reverse on the basis of an improper closing argument in Pirovolos, here, as in Pirovolos, there are factors that weigh in favor of the defendant, while others tip the scales in favor of the government. Certainly the inclusion of an improper, uninvited statement in the prosecution’s closing argument on rebuttal limited Chao’s opportunity to counter the argument without intervention by the court on proper motion. But, it is important to note that immediately following Chao’s objection to the closing argument, the trial court provided the jury with the following instruction: “It’s argument. If a lawyer makes an argument that isn’t supported by reasonable inference and the evidence, then the jury should disregard it.” Furthermore, at the conclusion of the case the court instructed the jury that: “Opening statements of counsel are for the purpose of acquainting you in advance with the facts counsel expect the evidence to show. Closing arguments of counsel are for the purpose of discussing the evidence.... “The evidence consists of the sworn testimony of the witnesses, the exhibits received in evidence, and stipulated or admitted facts.... “You are to consider only evidence received in this case. You should consider this evidence in the light of your own observations and experiences in life. You may draw such reasonable inferences as you believe to be justified from proved facts. “You are to disregard any evidence to which I sustained an objection or which I ordered stricken_ “You should decide this case solely on the evidence presented here in the courtroom. ” (Emphasis supplied). Thus, the court provided adequate limiting instructions to the challenged argument both immediately after the objection was made and again in the formal jury instructions given prior to the jury retiring for deliberation. The most important factor to be considered in determining whether the closing statement violated Chao’s rights is that the proof of Chao’s guilt was more than sufficient. As we noted in Pirovolos: “Most important, though, the evidence of Pirovolos's guilt was truly overwhelming. Such strong evidence of guilt ‘eliminates any lingering doubt that the prosecutor’s remarks unfairly prejudiced the jury’s deliberations.’ [United States v. Young, 470 U.S. 1, 19, 105 S.Ct. 1038, 1048, 84 L.Ed.2d 1 (1985) ]; see also Darden, 477 U.S. at 182, 106 S.Ct. at 2472 (overwhelming evidence of guilt a factor in affirming conviction despite serious prosecutorial misconduct); Berger v. United States, 295 U.S. 78, 89, 55 S.Ct. 629, 633, 79 L.Ed. 1314 (1935) (overwhelming evidence of guilt justified affirming verdict even in the face of highly improper conduct by prosecutor).” Pirovolos, 844 F.2d at 427. The evidence against Chao was overwhelming. Chao was actively involved in the operations of the Chicago warehouse business (Celi-Mar), the cocaine delivery point in July of 1987. Chao supervised and even participated in the unloading and loading of the drug cargo. Furthermore, Chao made statements at the time of this delivery to Peña and Tobon concerning what they might do to locate the boxes containing cocaine, the efforts the co-conspirators expended to keep Matteo Hernandez, the owner of the business, away from the premises during the cocaine delivery and expressed concern that the drug transfer be completed prior to Hernandez’ return. In addition, a search of the Celi-Mar warehouse following Chao’s arrest resulted in the discovery of a business card that Alvaro Quintero had instructed Peña to hand to Chao as a signal to Chao that he was to turn the cocaine delivery over to Peña. This card was probative because the evi-denee clearly established that Chao did, in fact, deliver cocaine to Peña as Quintero had contemplated when Peña exhibited the business card (signal). Even though it directly reflects on Chao’s credibility, the question of whether Chao stole a box of cocaine did not bear upon the central questions of whether Chao was an active and knowledgeable member of the conspiracy and further that he knowingly possessed cocaine with intent to distribute in July 1987. The overwhelming evidence of Chao’s guilt beyond a reasonable doubt is sufficient for us to conclude that the prosecutor's improper comments did not rise to the level of a violation of the defendant’s due process rights and thus deprive him of a fair trial. See Pirovolos, 844 F.2d at 427 (prosecutorial misconduct was not constitutional error in light of overwhelming evidence of defendant’s guilt). Likewise, the strong evidence of Chao’s guilt detailed in our consideration of the issue of constitutional error also leads us to the conclusion that Chao has failed to demonstrate “that any error here may have substantially influenced the jury’s verdict.” Pirovolos, 844 F.2d at 427. Thus, we hold that the prosecutor’s closing argument does not require reversal. Although we do not reverse Chao’s conviction, neither do we approve of the prosecutor’s conduct in closing argument. We strongly suggest that prosecuting attorneys thoroughly prepare their closing arguments before delivery and thus avoid such reckless and unsupportable comments and/or supposed legitimate inferences. “The prosecutor’s office is an entity [of the government] and as such it is the spokesman for the Government.” Giglio v. Unit- ed States, 405 U.S. 150, 154, 92 S.Ct. 763, 766, 31 L.Ed.2d 104 (1972). “The United States Attorney is the representative not of an ordinary party to a controversy, but of a sovereignty whose obligation to govern impartially is as compelling as its obligation to govern at all; and whose interest, therefore, in a criminal prosecution is not that it shall win a case, but that justice shall be done.” Berger v. United States, 295 U.S. 78, 88, 55 S.Ct. 629, 633, 79 L.Ed. 1314 (1935). “It is well understood in the realm of ethical and proper conduct of a criminal trial, that the prosecutor ‘may prosecute with earnestness and vigor — indeed, he should do so. But, while he may strike hard blows, he is not at liberty to strike foul ones.’ ” United States v. Chaimson, 760 F.2d 798, 809 (7th Cir.1985) (quoting Berger v. United States, 295 U.S. 78, 88, 55 S.Ct. 629, 633, 79 L.Ed. 1314 (1935)). Prosecutors must remember to live up to the code of professional ethics and fair play at all times or the American system of justice cannot endure, and ultimately our nation will lose confidence and trust in its rendering of justice, but, we refuse to reverse a conviction when we are convinced that the improper argument did not rise to the level of a constitutional violation of the defendant’s rights. As we concluded in Pirovo-los: “In closing, we emphasize that we do not reverse convictions to punish prosecutors. ‘It is better to punish the prosecutor directly; there is no lack of sanctions for a lawyer’s misconduct of which improper advocacy is a well recognized species.’ United States v. Mazzone, 782 F.2d 757, 763 (7th Cir.1986); See also Young, 470 U.S. at 7-11, 105 S.Ct. at 1042-44. When the seriousness of prose-cutorial misconduct and the weakness of evidence of guilt causes to question a trial’s fairness, we will not hesitate to reverse the resulting conviction and order a new trial. See, e.g., Mauricio v. Duckworth, 840 F.2d 454 (7th Cir.1988); Williams v. Lane, 826 F.2d 654 (7th Cir. 1987); United States v. Shue, 766 F.2d 1122 (7th Cir.1985). Here, however, ‘[t]he evidence against the appellant was overwhelming; it included substantial eye-witness evidence ... as well as physical evidence_ It is almost inconceivable that if the prosecutor had refrained from making the remarks that he did, the appellant [ ] would have been acquitted.’ Mazzone, 782 F.2d at 764. As the Supreme Court often has reminded us, the Constitution guarantees a fair trial, not a perfect one. See, e.g., Delaware v. Van Arsdall, 475 U.S. 673, 681, 106 S.Ct. 1431, 1436, 89 L.Ed.2d 674 (1986); United States v. Hasting, 461 U.S. 499, 508-09, 103 S.Ct. 1974, 1980-81, 76 L.Ed.2d 96 (1983); Bruton v. United States, 391 U.S. 123, 135, 88 S.Ct. 1620, 1627, 20 L.Ed.2d 476 (1968).” Pirovolos, 844 F.2d at 427. X. CONSCIOUS AVOIDANCE INSTRUCTION Luis Gonzalez asserts that the trial court erred when it provided a “conscious avoidance” jury instruction, alleging that the evidence “offered no reasonable basis for the inference that Gonzalez knew or should have suspected that something foul was afoot relating to the [July 1987 cocaine] shipment.” The trial court gave the following instruction: “You may infer knowledge from a combination of suspicion and indifference to the truth. If you find that a person had a strong suspicion that things were not what they seemed or that someone had withheld some important facts, yet shut his eyes for fear of what he would learn, you may conclude that he acted knowingly, as I have defined that word.” We have approved the above formulation of the “conscious avoidance” or “ostrich” instruction in a number of previous cases. See United States v. Valencia, 907 F.2d 671, 679 (7th Cir.1990); United States v. Paiz, 905 F.2d 1014, 1022-23 (7th Cir.1990); United States v. Defazio, 899 F.2d 626, 635-36 (7th Cir.1990); United States v. Herrero, 893 F.2d 1512, 1537-38 (7th Cir.), cert. denied, — U.S. -, 110 S.Ct. 2623, 110 L.Ed.2d 644 (1990); United States v. Talkington, 875 F.2d 591, 595-96 (7th Cir. 1989); United States v. Diaz, 864 F.2d 544, 549-51 (7th Cir.1988), cert. denied, 490 U.S. 1070, 109 S.Ct. 2075, 104 L.Ed.2d 639 (1989). In United States v. Bigelow, 914 F.2d 966, 970 (7th Cir.1990), we set forth the law currently applicable to the review of a conscious avoidance instruction: “At the outset, we note that in assessing the district court’s determination to give this instruction, we must review the evidence and any reasonable inferences that can be drawn from that evidence in the light most favorable to the government. See United States v. Talkington, 875 F.2d 591, 596 (7th Cir.1989); United States v. Johnson, 605 F.2d 1025, 1028 (7th Cir.1979). In addition, we do not review instructions in lonely isolation, but rather in the context of the trial as an integrated whole. See United States v. Herrero, 893 F.2d 1512, 1536 (and cases cited therein). “While we have warned that ‘this instruction should be given only when it addresses an issue reasonably raised by the evidence,’ [United States v. Diaz, 864 F.2d 544, 549 (7th Cir.1988) ], an ostrich instruction is appropriate when ‘the defendant claims a lack of guilty knowledge and there are facts and evidence that support an inference of deliberate ignorance.’ Talkington, 875 F.2d at 596 (quoting United States v. White, 794 F.2d 367, 371 (8th Cir.1988)) (and cases cited therein).” “In Diaz, we stated: ‘The ostrich instruction has been principally employed where there is evidence that the defendant is associated with a group, but where there is also evidence that the defendant consciously was avoiding knowledge of the illegal nature of the group’s activity. In most cases, the defendant acknowledges his association with the group, but, despite circumstantial knowledge to the contrary, denies knowledge of the group’s illegal activity.’ 864 F.2d at 550.” Viewing the evidence in the light most favorable to the government, on the challenge of the conscious avoidance instruction, there are a number of facts that lead us to the conclusion that Luis Gonzalez was aware of the illegal activities of the conspiratorial group. Daniel Petroski, who transported the cocaine in his truck, from Miami to New York and then to Chicago, identified Luis Gonzalez as the individual who met him at the Las Villas Warehouse in New York on July 28, 1987, entered his trailer and inspected his cargo with a flashlight. Gonzalez gave Petroski a yellow sheet of paper with the address and phone number of the Celi-Mar warehouse, told him to contact Chao and informed him that the load was misdirected and should have gone to Chicago. Gonzalez’ meeting with Petroski occurred during a trip Gonzalez made with co-conspirator, Tellechea, tracking the cocaine from Miami to New York and then to Chicago. In Chicago Tellechea and Gonzalez stayed at the same hotel and records of telephone calls between Telle-chea’s room and cocaine supplier Roberto Ramirez’ hotel room, as well as between Tellechea’s room and the Celi-Mar warehouse were received in evidence. When Gonzalez and Tellechea left Chicago, following Chao’s arrest, they flew together to Detroit and then on to Miami, in what Tellechea described to Julio Ortiz as an effort to erase trails. Luis Gonzalez’ activities relative to the July 1987 cocaine shipment took place following his appointment as manager of International Palm Products, the outlet for receiving the cocaine imported from Honduras. As reflected in his trial testimony, Julio Ortiz, a knowledgeable member of the conspiracy, formerly managed the business which received the imported cocaine. Although Gonzalez had headed International Palm Products for only two weeks, it is quite reasonable to believe that the person entrusted with taking over the role as head of a drug conspiracy’s importation outlet would be well aware of the conspiracy’s illegal activities. In light of the clandestine nature of a drug conspiracy and the control conspirators exercise over the organization’s membership, no conspiracy would place an individual at the very heart of its million-dollar drug import business if he was not well known and knowledgeable of its activities as well as trusted by the members of the organization. Based upon the totality of the evidence presented, a reasonable jury could have properly come to the conclusion that Luis Gonzalez was aware of the conspiracy and its illicit purposes. Despite the clear evidence of this association with members of the conspiratorial group, Luis Gonzalez asserts that he lacked knowledge of the conspiracy’s drug related activity. We have recognized that: “Such a scenario, one in which ‘the defendant acknowledges [his] association with the group but, despite circumstantial evidence to the contrary, denies knowledge of the group’s illegal activity,’ is a paradigm case for the use of the ‘ostrich’ instruction.” United States v. Paiz, 905 F.2d 1014, 1022 (7th Cir.1990) (quoting Diaz, 864 F.2d at 550). The conscious avoidance instruction also appropriately implements the legal rule that “with respect to the question of knowing participation in a conspiracy, it is well settled that ‘[i]f the facts indicate the defendant must have known something ... then a jury may be able to find beyond a reasonable doubt that [he] did know it, especially since the requirement of knowledge is satisfied by proof that the defendant willfully shut [his] eyes for fear of what [he] might see if [he] opened them, United States v. Josefik, 753 F.2d 585, 589 (7th Cir.1985).’ ” United States v. Diaz, 876 F.2d 1344, 1354 (7th Cir.1989) (quoting United States v. Cerro, 775 F.2d 908, 911 (7th Cir.1985)). We are convinced that the evidence in the record of Luis Gonzalez' involvement in the conspiracy demonstrates that his alleged lack of knowledge could only have resulted from his “cutting off of [his] normal curiosity” and thus is more than sufficient to uphold the trial court’s conscious avoidance instruction. XI. SUFFICIENCY OF THE EVIDENCE— THE CHALLENGES OF IZQUIERDO, AMEJEIRAS, LUIS GONZALEZ AND CHAO Izquierdo, Amejeiras, Luis Gonzalez and Chao challenge the sufficiency of the evidence to support their convictions on the conspiracy count (Count I). Chao alleges that there was inadequate proof that he knowingly possessed cocaine with intent to distribute (Count II), and Izquierdo, Ame-jeiras and Luis Gonzalez each argue that the insufficient documentation of their knowing participation in the conspiracy precludes their conviction under either a vicarious liability theory or an aiding and abetting theory for each of the two counts of possession of cocaine with intent to distribute (Counts II and III) alleged in the indictment. Chao claims that the evidence against him lacked the quantum of proof required to demonstrate that he possessed cocaine with intent to distribute (Count II). He further alleges that the insufficient evidence of his knowing participation in the conspiracy requires the conclusion that there was no basis for his conviction on the remaining count of possession of cocaine with intent to distribute (Count III) under either an aiding and abetting or vicarious liability theory. We have summarized our standard of review for sufficiency of the evidence as follows: “ ‘In evaluating [Haro’s] sufficiency of the evidence challenge, we note that [he] bears a heavy burden. Initially, we “review all the evidence and all the reasonable inferences that can be drawn from the evidence in the light most favorable to the government.” ’ United States v. Nesbitt, 852 F.2d 1502, 1509 (7th Cir. 1988) (quoting United States v. Pritchard, 745 F.2d 1112, 1122 (7th Cir.1984)). ‘The test is whether after viewing the evidence in the light most favorable to the government, “any rational trier of fact could have found the essential elements of the crime beyond a reasonable doubt.” ’ Pritchard, 745 F.2d at 1122 (quoting Jackson v. Virginia, 443 U.S. 307, 319, 99 S.Ct. 2781, 2789, 61 L.Ed.2d 560 (1979) (emphasis in original)).” United States v. Herrero, 893 F.2d 1512, 1531 (7th Cir.1990). As we observed in United, States v. Caudill, 915 F.2d 294, 297 (7th Cir.1990): Thus the appellants carry a heavy burden in asking us to reverse their convictions for lack of evidence. United States v. Caudill, 915 F.2d 294, 297 (7th Cir.1990). “A verdict will withstand a sufficiency of the evidence challenge unless there is no evidence from which the jury could find guilt beyond a reasonable doubt. See United States v. Beverly, 913 F.2d 337, 360 (7th Cir.1990); United States v. Durrive, 902 F.2d 1221, 1225 (7th Cir. 1990). In appeals of jury trials such as this case, we are obliged to ‘ “defer to reasonable inferences drawn by the jury and the weight it gave to the evidence. Likewise, we leave the credibility of witnesses, solely to the jury’s evaluation, absent extraordinary circumstances.” ’ Beverly, at 360 (quoting United States v. Hogan, 886 F.2d 1497, 1502 (7th Cir.1989) (citation omitted)). To sustain the conspiracy charge against [the defendants], the government need only prove the existence of the conspiracy and a participatory link with the defendants. Durrive, 902 F.2d at 1225, United States v. Missick, 875 F.2d 1294, 1297 (7th Cir.1989). Substantial evidence must support both the existence of the conspiracy and the defendants’ participation in it. Durrive, 902 F.2d at 1229. No