Full opinion text
Opinion for the court filed by Circuit Judge J. SKELLY WRIGHT. Concurring opinion filed by Circuit Judge HARRY T. EDWARDS. J. SKELLY WRIGHT, Circuit Judge: Title VII of the Civil Rights Act of 1964 proclaims one of this nation’s most fundamental, if yet unrealized, principles: a person shall not be denied full equality of employment opportunity on account of race, color, religion, sex, or national origin. Title VII bars both intentional discrimination and artificial, arbitrary, or unnecessary barriers to equal opportunity. In this case we review a decision of the United States District Court for the District of Columbia, Segar v. Civiletti, 508 F.Supp. 690 (D.D.C.1981), holding that the federal Drug Enforcement Agency (DEA) had engaged in a pattern or practice of discrimination against its black agents in violation of Title VIL A class comprising black agents initiated this suit in 1977 and the case came to trial in 1979. Finding that DEA had discriminated against black agents in salary, promotions, initial (GS) grade assignments, work assignments, supervisory evaluations, and imposition of discipline, 508 F.Supp. at 711-715, the District Court ordered a comprehensive remedial scheme consisting of a class-wide back-pay award, promotion goals and timetables to ensure that qualified black agents received promotions to the upper levels of DEA, and a class-wide frontpay award to compensate such qualified agents while they awaited the promotions they deserved. In the course of the proceedings the court also denied plaintiffs’ request for prejudgment interest and issued a preliminary injunction barring transfer or demotion of Carl Jackson (the Jackson injunction), a black agent who was the subject of adverse employment decisions immediately after his testimony for plaintiffs in this lawsuit. On appeal DEA challenges the liability determination, the remedial scheme, and the Jackson injunction. Plaintiffs cross-appeal the denial of prejudgment interest. As to the liability determination, DEA urges that the trial court erred in finding that plaintiffs had presented sufficient probative evidence to support any inference of discrimination at DEA, and urges that DEA had in any event effectively rebutted plaintiffs’ showing. As to the remedial scheme, DEA argues that class-wide relief was inappropriate and that imposition of promotion goals and timetables both exceeded the court’s remedial authority under Title VII and violated the equal protection component of the Fifth Amendment Due Process Clause. DEA also argues that Carl Jackson did not make a showing of retaliation sufficient to justify the preliminary injunction. To resolve this appeal we have had to plumb some of the deepest complexities of Title VII adjudication. After careful review, we affirm the District Court’s liability determination in its entirety. We also affirm the trial court’s decision to use a class-wide backpay remedy, but we vacate the backpay formula imposed and remand for reformulation of the particular backpay award. We also vacate the part of the District Court’s remedy that mandates promotion goals and timetables. We do not hold that such remedies exceed a court’s remedial authority under Title VII. Nor do we hold that such remedies violate the Constitution. Nonetheless, we find that the District Court’s particular order of goals and timetables was not appropriate on the current factual record. Because the front-pay remedy was specifically linked to the promotion goals and timetables, we vacate that part of the remedial order as well, and remand to the District Court for further consideration of appropriate remedies. We affirm the preliminary injunction against demotion or transfer of Carl Jackson and we expect the District Court to undertake resolution of the status of the Jackson injunction on remand. We affirm the trial court’s denial of prejudgment interest. I. Background DEA, an agency formed in 1973 within the Justice Department, enforces this nation’s federal criminal laws concerning the illegal sale, distribution, and use of drugs. Establishing DEA, the federal government sought to consolidate drug enforcement efforts that had theretofore been spread among several agencies. “Special agents” carry on the bulk of DEA’s criminal investigative work. DEA employs about 2,000 such agents, and as of 1978 seven percent were black. Special agents perform surveillance of suspected drug dealers, transact “buys” of drugs as evidence for prosecutions, do related undercover work, develop cases for prosecution by United States Attorneys, and, depending on their rank, supervise other special agents. Findings of Fact (Findings) ¶¶11-2, 508 F.Supp. at 693-695. The District Court made extensive findings of fact concerning DEA’s employment practices. See Findings ¶¶ 1-51, 508 F.Supp. at 692-711. Though we need not rehash the factual context of this case in its entirety, we will review the facts particularly pertinent to the issues on appeal. A. DEA’s Personnel Requirements Hiring. The Civil Service Commission Handbook establishes the minimum entry level requirements for special agents. Depending on qualifications, special agents will enter at either GS-7 or GS-9. The requirements for entry at GS-7 are three years of general experience and one year of specialized experience. The requirements for GS-9 are three years of general experience and two years of specialized experience. In addition, special agents are defined as criminal investigators, and this classification requires that one year of their prior specialized experience be in law enforcement or comparable work. Work Assignments. Special agents carry out the variety of assignments described above. Race influences the location of an agent’s assignment. All other things being equal, DEA will assign black agents to areas where a large percentage of the suspected violators are black. Race also influences the type of work agents receive. Black agents tend to perform a disproportionately large amount of undercover work. DEA generally infiltrates drug networks from the bottom up, and operates on the assumption that black agents will be more readily able to infiltrate organizations consisting primarily of blacks. The nature of an agent’s work assignments will have an important bearing on the agent’s prospects for promotion. Though some undercover work is desirable, a surfeit of such work injures an agent’s promotion opportunities because the agent is unable to obtain the breadth of experience needed for promotions. Findings 11 23, 508 F.Supp. at 705. Promotions. At DEA promotions from GS-7 to GS-9, from GS-9 to GS-11, and from GS-11 to GS-12 are noncompetitive. A special agent receives a promotion upon completion of one year of service in grade, recommendation by the agent’s group supervisor, concurrence by a second level supervisor, and approval by a DEA regional director. Promotions from GS-12 up through GS-18, the highest GS level at DEA, are competitive agency-wide. To receive such a promotion an agent must satisfy the minimum in-grade requirement, be placed on the “best qualified” list by the appropriate rating and ranking board, and be selected by the appropriate selecting official. In making determinations the rating and ranking boards rely primarily on the agent’s most recent performance appraisal, information on disciplinary action within the last two years, and the agent’s application and profile sheet. Those agents chosen for the best qualified list are then ranked numerically on a series of performance factors. Rating and ranking boards have not been provided with any particular guidance for assigning numerical values to various aspects of an agent’s performance. Findings 114, 508 F.Supp. at 695. B. This Lawsuit In January 1977 two black special agents of DEA, and an association representing all black special agents, brought suit alleging that DEA had engaged in a pattern or practice of racial discrimination against black special agents in violation of Title VII of the Civil Rights Act of 1964, 42 U.S.C. § 2000e et seq., as amended by the Equal Employment Opportunity Act of 1972, 42 U.S.C. § 2000e-16 (1976 & Supp. V 1981). These agents alleged discrimination in recruitment, hiring, initial grade assignments, salary, work assignments, evaluations, discipline, and promotions. See Complaint, Joint Appendix (JA) 22. On September 9, 1977 the trial court, pursuant to Federal Rule of Civil Procedure 23(b)(2), certified the class of all blacks who then served or had had been discharged as special agents at DEA, and who had applied for positions or would in the future apply. Order of Class Certification, September 9, 1977, JA 37. Before trial the parties settled the claims involving discriminatory recruitment and hiring, but could not come to terms on the other issues. See Stipulation of Settlement of Plaintiffs’ Claims of Discrimination in Recruiting and Hiring of Special Agents, JA 44. As is common in Title VII class actions, the District Court bifurcated the trial into separate liability and remedial phases. After lengthy discovery, the liability issues came to trial in April 1979. The trial was in large measure a duel of experts armed with sophisticated statistical means of proof. 1. The plaintiffs’ case. The plaintiffs presented a range of statistical and anecdotal evidence of discrimination. The statistical evidence included several multiple linear regression analyses as well as a number of studies considering the effects of particular employment practices. Multiple regression is a form of statistical analysis used increasingly in Title VII actions that measures the discrete influence independent variables have on a dependent variable such as salary levels. See Valentino v. U.S. Postal Service, 674 F.2d 56, 70 (D.C.Cir.1982). Typically the independent variables in Title VII cases will be race, age, education level, and experience levels. The first step in a multiple regression analysis is specification of the independent (or explanatory) variables thought likely to affect significantly the dependent variable. The choice of proper explanatory variables determines the validity of the regression analysis. A coherent theory, devised prior to observation of the particular data, must be employed to select the relevant explanatory variables. See Vuyanich v. Republic Nat’l Bank of Dallas (Vuyanich I), 505 F.Supp. 224, 269 (N.D.Tex.1980), vacated on other grounds, 723 F.2d 1195 (5th Cir.1984). When the proper variables have been selected, the multiple regression analysis is conducted, generally by a computer. In essence, the regression measures the impact of each potential explanatory variable upon the dependent variable by holding all other explanatory variables constant. The analysis yields figures demonstrating how much of an observed disparity in salaries can be traced to race, as opposed to any of the other potential explanatory variables. The computer analysis will generally also yield two other measurements that assist in evaluation of the explanatory power of the regression. The first is “T-Ratio.” The T-Ratio measures the probability that the result obtained could have occurred by chance. The second is R2. The R2 figure measures, to a certain extent, the degree to which a multiple regression analysis taken as a whole explains observed disparities in a dependent variable. Having observed an average disparity in salary of about $3,000 between white and black special agents at DEA, plaintiffs’ experts, Professors Bergmann and Straszheim, formulated a regression analysis to discover whether and to what extent race explained the observed salary disparity. The experts based their analysis on a “human capital model.” A widely accepted approach, the model builds on labor economists’ findings that the human capital an employee brings to a job — such as education and experience—in large measure determines the employee’s success. See Note, Beyond the Prima Facie Case in Employment Discrimination Law: Statistical Proof and Rebuttal, 89 Harv.L.Rev. 387, 408 n. 90 (1975); Vuyanich I, supra, 505 F.Supp. at 265-267. Plaintiffs’ experts selected education, pri- or federal experience, prior nonfederal experience, and race as the four independent variables that might explain the salary differential. Information regarding these independent variables came from the computerized JUNIPER personnel information tapes of the Department of Justice. Findings 117a-b, 508 F.Supp. at 696. Professors Bergmann and Straszheim then ran the regressions. They first evaluated the causes of salary disparities among all agents as of five dates: the first of January in 1975, 1976, 1977, and 1978 and the first of October in 1978. This study generated the following results: RACE DATE COEFFICIENT T-RATIO 1/1/75 -$1,628 4.65 1/1/76 -$1,744 5.37 1/1/77 -$1,119 5.15 1/1/78 -$1,934 5.15 10/1/78 -$1,877 4.50 Findings 117b, 505 F.Supp. at 696. The race coefficient measures the salary disparities between white and black agents when education and prior experience are held constant. The T-Ratio figures here correspond to standard deviations of four or five. See D. Baldus & J. Cole, Statistical Proof of Discrimination 297 n. 14 (1980) (hereinafter “D. Baldus & J. Cole”). Since a standard deviation level higher than three indicates that the odds are less than one in a thousand that an observed result could have occurred by chance, these figures indicate that the odds are far less than one in a thousand that the observed disparities for any year could have occurred by chance. A study is generally considered to be statistically significant when the odds that the result occurred by chance are at best one in 20. See D. Baldus & J. Cole, supra, at 297. Professors Bergmann and Straszheim then ran a second regression to measure salary disparities over the same time frame for agents hired after 1972. They intended this study to generate some measure of the effects of race discrimination at DEA after 1972. Title VII applies to DEA in this action only as of that date. Because the first regression measured disparities in the salaries of all black agents, including those hired before 1972, the race coefficient in that study may have reflected disparities resulting from the continuing effects of discrimination that occurred prior to 1972, rather than actionable post-1972 discrimination. This second regression generated the following results: RACE DATE COEFFICIENT T-RATIO 1/1/75 -$ 378 .84 1/1/76 -$1,864 2.54 1/1/77 -$1,119 3.18 1/1/78 -$ 866 2.07 10/1/78 -$1,026 2.30 Findings IT 7d, 508 F.Supp. at 696. Again a significant salary disparity between agents with comparable education and experience was revealed. The T-Ratios indicate that for every year, save 1975, the possibility that the result could have occurred by chance was at most one in 20. Though these figures are not at as high a level of significance as were those of the first regression, they still meet the generally accepted test for statistical significance. The second regression, moreover, tends to understate the amount of post-1972 discrimination at DEA. Because the post-1972 study measures discrimination among newer agents, the study focuses on the speed with which the new recruits make their way through the lower levels of DEA. Promotions at these levels are relatively automatic, and discrimination thus has less opportunity to work its effects. Discrimination will most adversely affect older agents contending for upper level positions; promotion decisions at these levels incorporate far more discretionary elements and leave more room for bias. See Part I-A supra. The study does not measure any post-1972 discrimination against those hired before 1972. Since these agents would have been the ones contending for the upper level positions during the time frame studied, they would have been the ones on whom discrimination would have been most likely to operate. The problem is particularly severe with respect to the 1975 race coefficient. Almost half of those studied to obtain this figure were hired in 1974. Findings ¶ 7c, 508 F.Supp. at 696. Since they were in their first year at the time of the study, they would not yet have been eligible for a grade promotion. Having uncovered evidence of significant discrimination in salary levels, plaintiffs’ experts undertook a more exacting inquiry into DEA’s employment practices to pinpoint where discrimination was taking place. They first examined DEA’s initial grade assignment practices. Through regression analyses they determined at a sufficient level of statistical significance that blacks were 16 percent less likely than comparably qualified whites to have been hired at GS-9 rather than GS-7. For those hired after 1972, blacks were 12 percent less likely to be hired at GS-9. Findings ¶ 9, 508 F.Supp. at 698-699. The experts then evaluated work assignments, supervisory evaluations, and discipline. In all three categories statistical analysis revealed significant levels of discrimination against black agents. Findings ¶¶ 12, 14, 15, 508 F.Supp. at 698-700. Finally, plaintiffs’ experts studied promotions at DEA. Promotions up to the GS-11 level were found to be relatively automatic. The promotion rate from GS-11 to GS-12 was 70 percent for blacks and 82 percent for whites. This differential met generally accepted levels of statistical significance. Differentials in promotion rates for positions above GS-12 were also found, but— largely because of the small sample size— these differentials did not achieve statistical significance at generally accepted levels. Findings 1116, 508 F.Supp. at 701-702. To buttress the statistical proof plaintiffs introduced anecdotal testimony of discrimination. This evidence consisted of accounts by several black agents of perceived discrimination against them in initial grade assignments, work assignments, supervisory evaluations, and discipline. These agents also testified about their general perceptions of racial hostility at DEA. 2. Defendant DEA’s case. DEA responded to plaintiffs’ case in several ways. The rebuttal consisted of expert testimony attacking the methodological integrity and explanatory value of plaintiffs’ statistics, alternative statistical analyses tending to show an absence of discrimination, testimonial evidence concerning DEA’s equal employment opportunity programs, and cross-examination of plaintiffs’ anecdotal accounts of individual discrimination. DEA’s first expert was Dr. J. Wanzer Drane, an associate professor of statistics at Southern Methodist University. In his testimony he attacked the methodology of plaintiffs’ statistical analyses. He asserted that plaintiffs had failed in their analysis to account for the relevant explanatory variable of prior law enforcement experience, and that the explanatory power of plaintiffs’ salary studies (the R2 and the T-Ratio) was too low. He also presented a more generalized critique of the suitability of the regression methodology for measurement of discrimination in the present situation. Findings II 7f-n, 508 F.Supp. at 696-697; see Trial Transcript (Tr.) 1850-1871; brief for appellants at 36-38. To supplement this critique DEA offered an alternative statistical analysis that tended to show an absence of discrimination. This study — prepared by Dr. B.C. Spradlin, a consultant — was not a regression but an alternative statistical methodology known as cohort analysis. Under this approach all employees who start together at the same level are surveyed over the course of an observation period and their comparative progress in salary and promotion is evaluated. Evaluating promotion and salary disparities among DEA agents who started at the same year and grade level, Dr. Spradlin’s cohort analysis suggested significant discrimination in four of 15 groups. These four groups were broken into subgroups. Discrimination was found in two of the subgroups. DEA then examined the personnel files of those in the subgroups showing discrimination and found that three individuals had been misclassified. When these three were properly classified, the study showed no significant discrimination. Findings ¶ 8, 508 F.Supp. at 697-698; see Tr. at 1909-1913. Testimonial evidence buttressed DEA’s statistical rebuttal. DEA presented extensive general testimony on its efforts to establish equal opportunity programs and implement equal opportunity goals at the agency. Through cross-examination of plaintiffs’ witnesses, DEA also sought to rebut every particular anecdotal account of discrimination. C. The District Court Decision 1. The liability determination. Judge Robinson held that DEA had discriminated against black special agents in violation of Title VII across a range of employment practices. The court found that the salary differentials between white arid black agents were a result of race discrimination, and that DEA had discriminated against black agents in grade-at-entry, work assignments, supervisory evaluations, and promotions. 508 F.Supp. at 712-715. The finding of discrimination in promotions extended to promotions above the GS-12 level, even though the court did not credit plaintiffs’ statistical evidence of discrimination at that level because the statistics had not achieved acceptable levels of statistical significance. The court based its finding of discrimination at the upper levels on inferences from proven discrimination at the immediately preceding levels and discrimination in the factors that bear most directly on promotions (work assignments, evaluations, and discipline). 508 F.Supp. at 714-715. To make these deterriiinations the District Court credited the bulk of plaintiffs’ statistical evidence and rejected both DEA’s critique of this evidence and DEA’s alternative statistics. In particular, the court rejected DEA’s claims that plaintiffs’ statistics did not possess sufficient explanatory power, that the failure to account for prior law enforcement experience skewed the studies, and that DEA’s cohort analysis rebutted plaintiffs’ showing of discrimination. As to the alleged failure to account for law enforcement experience, the court held that DEA’s objection was “speculative and incapable of rebutting plaintiffs' statistical showing.” Id. at 712. Moreover, the court found that the “cohort analysis was irreparably flawed,” primarily because the methodology focused on groups too small to generate statistically significant evidence of discrimination. Id. at 698, 712. Since DEA had hinged its defense on this effort to rebut plaintiffs’ showing of race-related disparities, the District Court’s findings were dispositive against DEA. 2. The remedies determination. Having found pervasive discrimination at DEA, the District Court — in a separate remedial proceeding — set out to formulate an appropriate remedial plan. The essential elements of the plan were class-wide backpay, promotion goals and timetables, and class-wide frontpay. See Memorandum Opinion (Mem.Op.) and Order (Remedial Order), February 17, 1982, JA 114. Class-wide Backpay. Rather than order individualized relief hearings, see Int’l Brhd of Teamsters v. United States, 431 U.S. 324, 361-362, 97 S.Ct. 1843, 1867-1868, 52 L.Ed.2d 396 (1977), the District Court ordered a class-wide award of backpay for members of the plaintiff class. For successive one-year periods beginning in July 1972, a class-wide backpay pool figure would be calculated. The calculations would derive from plaintiffs’ first salary regression study (which measured disparities among all agents including those hired before 1972). For every year for which figures were available — 1975 to 1979 — the class-wide pool figure would be the race coefficient multiplied by the number of black special agents. For the years before 1975 and after 1979 the race coefficient would be derived by extrapolating backward and forward from the available figures, and this extrapolated coefficient would be multiplied by the number of black agents. The annual backpay pool would be distributed evenly among eligible black agents. Only agents above the GS-9 level during the year in question were made eligible. The court excluded agents at GS-7 and GS-9 because most discrimination was found to occur at the higher levels of DEA. The court did, however, permit individual plaintiffs to come forward and seek backpay for discrimination suffered in initial grade assignment (viz. assignment to GS-7 instead of GS-9). Any such individual awards would be subtracted from the class-wide pool in order to prevent double liability. Mem.Op. at 3, JA 116. Promotion Goals and Timetables. Finding discrimination at the upper levels of DEA, the District Court ordered remedial promotion goals and timetables. Since black agents made up at least 10 percent of agents at every level through GS-12 the court held that a 10 percent goal was appropriate for all levels above GS-12. Mem.Op. at 4, JA 117. To meet this goal the court ordered DEA to promote one black agent for every two white agents until 10 percent black representation had been met at GS-13 and above (or until five years had passed). Id. Class-wide Frontpay. To compensate black agents awaiting promotion under the goals and timetables plan the court established a class-wide frontpay formula. Frontpay pool calculations were also based on extrapolations from the salary regression, but the pool was to be adjusted to reflect progress DEA had made under the promotions goals and timetables. Remedial Order at 9-12, JA 126-129. The pool was to be distributed to all black agents at GS-12 for at least two years and all black agents above GS-12. 3. Other issues. In the course of the proceeding two other issues arose. Plaintiffs sought and were refused an award of prejudgment interest on the backpay awards. See Mem.Op. at 3 n. 4, JA 116 n. 4. Also, during the time between the liability and remedial determinations the court issued a preliminary injunction barring demotion or transfer of' black special agent Carl Jackson. See Memorandum Opinion and Order of May 5, 1981, JA 101. Shortly after Jackson had testified at trial in this case he became the target of harassment and eventually of .adverse employment actions including demotion and transfer. The District Court concluded that there was a high likelihood that these actions were in retaliation for Jackson’s testimony, and therefore preliminarily enjoined Jackson’s demotion or transfer. D. The Appeal. DEA appeals several aspects of both the liability and the remedial determinations, and also challenges the Jackson injunction. Plaintiffs cross-appeal from the denial of prejudgment interest. We will consider separately DEA’s challenges to the liability decision, the remedial decision, and the Jackson injunction. We will then consider plaintiffs’ claim for prejudgment interest. II. The Liability Determination A. Framework for Analysis A plaintiff in a Title VII action can prove liability under two theories: disparate treatment or disparate impact. In a disparate treatment claim a plaintiff seeks to prove that an employer intentionally “treats some people less favorably than others because of their race, color, religion, sex, or national origin.” Teamsters, supra, 431 U.S. at 335 n. 15, 97 S.Ct. at 1854 n. 15. Proof of illicit motive is essential, but, especially in cases alleging class-wide discrimination, illicit motive may be inferred from a sufficient showing of disparity between members of the plaintiff class and comparably qualified members of the majority group. Id. Such class-wide allegations of discrimination are commonly referred to as “pattern or practice” cases. In Teamsters, supra, the Supreme Court noted that “the question whether the company engaged in a pattern or practice of discriminatory [action] * * * involves controlling legal principles that are relatively clear,” id. at 334-335, 97 S.Ct. at 1854, and went on to characterize the case as one of disparate treatment. A claim that the sum of an employer’s practices results' in less favorable treatment of members of the plaintiff class than of comparably qualified whites or males may justify an inference that “discrimination was the company’s standard operating procedure — the regular rather than the unusual practice.” Id. at 336, 97 S.Ct. at 1855. This is because a disparity in treatment of the comparably qualified is “the expected result of a regularly followed discriminatory policy.” Id. at 361 n. 46, 97 S.Ct. at 1867 n. 46. In a disparate impact claim a plaintiff challenges “employment practices that are facially neutral in their treatment of different groups but that in fact fall more harshly on one group than another and cannot be justified by business necessity.” Id. at 336 n. 15, 97 S.Ct. at 1855 n. 15. To prevail on a disparate impact theory a plaintiff need not prove illicit motive; “Congress directed the thrust of the Act to the consequences of employment practices, not simply the motivation.” Griggs v. Duke Power Co., 401 U.S. 424, 432, 91 S.Ct. 849, 854, 28 L.Ed.2d 158 (1971) (emphasis in original). This disparate impact concept may be relevant in two ways to a case involving allegations of class-wide discrimination. First, in addition to bringing a pattern or practice disparate treatment claim, plaintiffs may well challenge the disparate impact of specific employment practices and thus force the employer to prove the job-relatedness of those practices. See Griggs, supra, 401 U.S. at 432, 91 S.Ct. at 854. Second, plaintiffs’ pattern or practice disparate treatment challenge to the employment system as a whole may also implicate disparate impact analysis. A pattern or practice disparate treatment case shares with a typical disparate impact suit the allegation that an employer’s practices have had a systemic adverse effect on members of the plaintiff class. See Teamsters, supra, 431 U.S. at 336 n. 15, 97 S.Ct. at 1855 n. 15 (“Either theory may, of course, be applied to a particular set of facts.”); Vuyanich v. Republic Nat’l Bank of Dallas (Vuyanich II), 521 F.Supp. 656, 663 (N.D.Tex.1981), vacated on other grounds, 723 F.2d 1195 (5th Cir.1984). Though a plaintiff class will initially seek to show a disparity among the comparably qualified in order to prove disparate treatment, an employer may seek to defend by pointing to a specific, arguably nondiscriminatory, employment practice as the cause of the observed disparity. In such situations the defendant may appropriately be required to demonstrate the business necessity of the practices causing the disparity because the court will have before it all the elements of a traditional disparate impact claim. See Part II-A-2 infra. Proceeding under the disparate treatment theory, plaintiffs in this case allege a pattern or practice of illegal discrimination at DEA. Proceeding under the disparate impact theory, plaintiffs also challenge a number of DEA’s specific employment practices — initial grade assignments, work assignments, supervisory evaluations, discipline, and promotions decisions. Thus plaintiffs challenge both the entirety of DEA’s employment system and several specific components of that system. To establish a framework for analysis of these allegations, we will first examine the sequence and allocation of proof in a class action alleging a pattern or practice of disparate treatment and alleging disparate impact with respect to specific employment practices. We will then locate within that framework the approach that the parties have taken in joining the liability issue in this case. 1. Sequence and allocation of proof. Functionally the disparate treatment and disparate impact models have different aims, and the proof sequences associated with each reflect these differences. Disparate treatment aims at discovery and elimination of intentional discrimination. On the disparate treatment claim the “ultimate burden of persuading the trier of fact that the defendant intentionally discriminated against the plaintiff remains at all times with the plaintiff.” Texas Dep’t of Community Affairs v. Burdine, 450 U.S. 248, 253, 101 S.Ct. 1089, 1093, 67 L.Ed.2d 207 (1981). Since the plaintiff class will likely try to meet this ultimate burden by proving a disparity sufficient to permit an inference of discrimination, plaintiff must carry the burden of persuasion as to the existence of the disparity. Disparate impact aims at discovery and elimination of facially neutral employment practices that adversely affect minorities and cannot be justified as necessary to an employer’s business. On the disparate impact claim plaintiffs bear the burden of persuasion as to the existence of a race-related disparity caused by an employment practice, but, once plaintiffs have made this showing, the employer bears the burden of persuasion as to the business necessity of the practice. See Vuyanich II, supra, 521 F.Supp. at 660. Though allocations of proof differ in this crucial way, an important point of convergence exists in class actions like the present case. Both pattern or practice disparate treatment claims and disparate impact claims are attacks on the systemic results of employment practices. The pattern or practice claim amounts to an allegation that an observed disparity is the systemic result of an employer’s intentionally discriminatory practices. The disparate impact claim amounts to an allegation that an observed disparity is the systemic result of a specific employment practice that cannot be justified as necessary to the employer’s business. Consequently the proof of each claim will involve a showing of disparity between the minority and majority groups in an employer’s workforce. These two factors — the difference in ultimate burden of proof and the similarity in proof of disparity — must inform our understanding of the intermediate burdens that the parties to such an action face. A plaintiff class seeking to show a pattern or practice of disparate treatment must “carry the initial burden of offering evidence adequate to create an inference that” employment decisions were “based on a discriminatory criterion illegal under the Act.” Teamsters, supra, 431 U.S. at 358, 97 S.Ct. at 1866. This usually means providing evidence—often in statistical form — of a disparity in the position of members of the plaintiff class and comparably qualified whites. Similarly, on the disparate impact challenges to specific employment practices the plaintiff class must present evidence that the practices have a disproportionately adverse effect on the plaintiffs. How far this prima facie showing will carry the plaintiff toward its ultimate burden of persuasion depends on both the strength of the plaintiffs’ evidence and the nature of the defendant’s response. A defendant must tailor its response to the nature of the plaintiff’s proof. Teamsters, supra, 431 U.S. at 360 n. 46, 97 S.Ct. at 1867 n. 46. In general, though, the response will follow two paths. The employer can endeavor to refute the plaintiffs’ claim that a disparity exists. Alternatively, the employer can offer an explanatory defense; such a defense amounts to a claim that an observed disparity has not resulted from illegal discrimination. The nature of these defenses, and the burdens associated with them, differ in important ways. a. Challenges to the plaintiffs’ proof Challenging the accuracy or significance of plaintiffs’ proof, a defendant seeks to show that the alleged disparity on which plaintiffs’ case is bottomed does not exist. Such a defense can of course be raised against both a disparate treatment and a disparate impact claim. Typically the challenge will focus on the integrity of the plaintiffs’ statistical methodology and the significance of the results shown. Often the defendant will supplement the critique with alternative statistical analyses tending to refute the plaintiffs’ evidence of disparity. The defendant need not carry the burden of persuasion as to the nonexistence of a disparity; on the disparate treatment claim “the ultimate burden of persuading the trier of fact that the defendant intentionally discriminated against the plaintiff remains at all times with the plaintiff,” Burdine, supra, 450 U.S. at 253, 101 S.Ct. at 1093, and on the disparate impact claims the plaintiffs must persuade the trier of fact that a disparity exists. The defendant’s rebuttal must, however, at least raise a genuine issue of material fact concerning the accuracy of the picture painted by the plaintiffs’ statistics. And introduction of evidence sufficient to raise a genuine issue of material fact does not necessarily vindicate the defendant. Burdine, supra, 450 U.S. at 254-255, 101 S.Ct. at 1094. Rather, the strength of the evidence the defendant must produce to prevent the plaintiff from carrying the burden of persuasion as to disparity depends, as in any case, on the strength of the plaintiffs’ proof. “[T]he defendant's evidence must do more than merely raise an issue of fact. * * * It must cast sufficient doubt on the plaintiff’s proof to cause the trier of fact to conclude that the plaintiff has not proved discrimination by a preponderance of the evidence.” Vuyanich II, supra, 521 F.Supp. at 663. b. The explanatory defense. Alternatively, a defendant can attempt to show that any observed disparities between plaintiffs and the majority group did not result from discrimination violative of Title VII. The requirements of such a rebuttal will vary according to the type of claim the defendant seeks to rebut. To rebut a disparate impact challenge to a specific employment practice causing a disparity the employer must prove the business necessity of the practice. See Albemarle Paper Co., supra, 422 U.S. at 425, 95 S.Ct. at 2375. To rebut a disparate treatment challenge the employer can argue that the observed disparity between the plaintiff class and the majority group does not support an inference of intentional discrimination because there is a legitimate, nondiscriminatory explanation for the disparity. For example, the defendant might come forward with some additional job qualification—not sufficiently perceptible to plaintiffs to have permitted them to account for it in their initial proof—that the plaintiff class lacks, thus explaining the disparity. The nature of the burden that the defendant bears on such a defense is not entirely free of doubt. The defendant must at least make a “clear and reasonably specific showing,” based on admissible evidence, that the alleged nondiscriminatory explanation in fact explains the disparity. Burdine, supra, 450 U.S. at 253-255, 101 S.Ct. at 1093-1095. In the context of an individual plaintiff’s claim of disparate treatment, the Court in Burdine suggested that a defendant need do no more than make such an articulation. Id. at 253-254, 101 S.Ct. at 1093-1095. Though the principles on which Burdine is based are fully applicable to pattern or practice cases, the specific definition of the rebuttal burden on an employer in an individual plaintiff’s disparate treatment case should not be unthinkingly applied to class actions such as the present case. In an individual case a defendant’s nondiscriminatory explanation serves only to undermine the inference of discriminatory intent arising from plaintiffs proof. Since plaintiff bears the burden of persuasion on the issue of intentional discrimination, the defendant should not have to persuade the trier of fact that plaintiff was treated less well for a nondiscriminatory reason. The defendant is required only to present evidence sufficient to permit the trier of fact legitimately to decline to infer discrimination from the plaintiffs proof. In the context of an individual’s suit, the bare articulation of a legitimate nondiscriminatory explanation generally suffices to undermine a plaintiff’s initial proof in this way because the plaintiff’s prima facie case will typically consist of the low-threshold showing of McDonnell Douglas Corp. v. Green, 411 U.S. 792, 93 S.Ct. 1817, 36 L.Ed.2d 668 (1973). When a defendant in a pattern or practice class action offers such an explanation, the circumstances differ in two crucial ways. First, to make an initial showing of disparate treatment in such eases the plaintiff class will typically have presented statistical evidence showing pervasive disparities and eliminating most, if not all, potential nondiscriminatory explanations for the observed disparities. See Vuyanich II, supra, 521 F.Supp. at 663; Part II-A-1 supra. Though the employer is not required to meet a burden of persuasion in rebutting the disparate treatment claim, the nondiscriminatory explanation must cast sufficient doubt on the plaintiffs’ proof to permit the trier of fact legitimately to decline to draw an inference of discrimination from that proof. The bare articulation of a nondiscriminatory explanation, while sufficient to rebut an individual plaintiff’s low-threshold McDonnell Douglas showing, generally will not suffice as a rebuttal to a typical class-wide showing of pervasive discrimination. Burdine made this much clear: In saying that the presumption [of discrimination] drops from the case, we do not imply that the trier of fact no longer may consider evidence previously introduced by the plaintiff to establish a prima facie case. A satisfactory explanation by the defendant destroys the legally mandatory inference of discrimination arising from the plaintiff’s initial evidence. Nonetheless, this evidence and inferences properly drawn therefrom may be considered by the trier of fact on the issue of whether the defendant’s explanation is pretextual. Indeed, there may be some cases where the plaintiffs initial evidence, combined with effective cross-examination of the defendant, will suffice to discredit the defendant’s explanation. 450 U.S. at 255 n. 10, 101 S.Ct. at 1095 n. 10 (emphasis added). The typical pattern or practice case is closely analogous to the situation the Court envisions in the last sentence of the quoted passage:, the plaintiffs’ initial offer of evidence will have been so strong that the bare articulation of a nondiscriminatory explanation will not suffice to rebut it. Thus in both individual and class action contexts the defendant faces the same rebuttal burden; it must present sufficient evidence to permit the trier of fact to decline to draw the inference of discrimination from the plaintiffs’ proof. But in the class action pattern or practice case the strength of the evidence sufficient to meet this rebuttal burden will typically need to be much higher than the strength of the evidence sufficient to rebut an individual plaintiffs low-threshold McDonnell Douglas showing. Second, the employer’s effort to rebut the pattern or practice claim by articulating a legitimate nondiscriminatory explanation may have the effect of putting before the court all the elements of a traditional disparate impact case. By its explanation of an observed disparity the employer will typically pinpoint an employment practice (or practices) having a disparate impact on a protected class. And to rebut plaintiffs’ case the employer will typically be required to introduce evidence showing that the employment practice in fact caused the observed disparity. See Burdine, supra, 450 U.S. at 258, 101 S.Ct. at 1096 (“defendant will normally attempt to prove the factual basis for its explanation”). In this situation, between the plaintiffs’ prima facie showing of disparity and the defendant’s rebuttal explanation of the disparity, the essential elements of a disparate impact case will have been placed before the trier of fact. Such a case is ripe for resolution using disparate impact analysis. Though the plaintiffs in a disparate treatment case bear the burden of persuasion as to the existence of a disparity, the defendant bears the burden of proving the business necessity of the practices causing the disparity. Albemarle Paper Co., supra, 422 U.S. at 425, 95 S.Ct. at 2375. Thus when an employer defends a disparate treatment challenge by claiming that a specific employment practice causes the observed disparity, and this defense sufficiently rebuts the plaintiffs’ initial case of disparate treatment, the defendant should at this point face a burden of proving the business necessity of the practice. Accord Vuyanich II, supra, 521 F.Supp. at 662-663; see Bartholet, Application of Title VII to Jobs in High Places, 95 Harv.L.Rev. 945, 1004-1006 (1982). The only difference between this situation and the traditional disparate impact case is that in the latter the plaintiff articulates the employment practice causing the adverse impact and forces the employer to defend it, while in the former the employer articulates the employment practice and must then go on to defend it. Accord Vuyanich II, supra, 521 F.Supp. at 663. Some case law from other circuits has expressed a reluctance to apply disparate impact analysis in this situation. Two concerns fuel this reluctance: (1) the perceived unfairness of placing on the defendant the dual burden of articulating which of its employment practices caused the adverse impact at issue and proving the business necessity of the practice, see, e.g., Pouncy v. Prudential Ins. Co. of America, 668 F.2d 795, 800 (5th Cir.1982) (plaintiff required to point to specific employment practice causing adverse impact “in order to allocate fairly the parties’ respective burdens of proof at trial”); and (2) the risk that an employer will be forced to justify the entire range of its employment practices when a plaintiff shows only that a disparity exists, see Rivera v. City of Wichita Falls, 665 F.2d 531, 539 (5th Cir.1982). These concerns are, however, unpersuasive and difficult to harmonize with the purposes of Title VII. As a practical matter, this issue arises in a context that renders such concerns largely irrelevant. An employer will face the justificatory burden only after a plaintiff class has shown a disparity in the positions of members of the class and the majority group who appear to be comparably qualified; if plaintiffs fail to make their prima facie case, the employer never faces this justificatory burden. But if the plaintiffs have made their prima facie case, the employer, to avoid liability under the disparate treatment theory, will have to advance some nondiscriminatory explanation for the disparity. An employer’s claim that it cannot isolate the cause of the disparity will be unlikely to deflect the force of the inference of discrimination from plaintiffs’ proof. The defendant will in all likelihood point to a specific job qualification or performance/evaluation rating as the explanation for the observed disparity. Thus application of disparate impact in this situation will not, the fears of the Pouncy court notwithstanding, place on the employer any additional burden of articulation; to rebut the disparate treatment claim the employer will have had to articulate which employment practices cause an observed disparity. Nor will the employer be forced to justify all of its employment practices. The employer will be required to show the job relatedness of only the practice or practices identified as the cause of the disparity- The concerns of Pouncy and Rivera are no more compelling on the theoretical level. The employer will possess knowledge far superior to that of the plaintiff as to precisely how its employment practices affect employees. This fact, as the Fifth Circuit noted in Pouncy, 668 F.2d at 801, traditionally justifies placing on the defendant the burden of proving the business necessity of an employment practice. So too it justifies the lesser burden of requiring the employer to articulate which of its employment practices adversely affect minorities. A rule placing this justificatory burden on the employer advances the purposes of Title VII far better than would the contrary result. “What is required by Congress is the removal of artificial, arbitrary, and unnecessary barriers to employment when the barriers operate invidiously to discriminate on the basis of racial or other impermissible classification.” Griggs, supra, 401 U.S. at 430-431, 91 S.Ct. at 853; accord Connecticut v. Teal, 457 U.S. 440, 451, 102 S.Ct. 2525, 2533, 73 L.Ed.2d 130 (1982) (“Title VII strives to achieve equality of opportunity by rooting out ‘artificial, arbitrary, and unnecessary’ employer-created barriers to professional development”); Teamsters, supra, 431 U.S. at 364, 97 S.Ct. at 1869 (“a primary objective of Title VII is prophylactic: to achieve equal employment opportunity and to remove barriers that have operated to favor white male employees over other employees”); Albemarle Paper Co., supra, 422 U.S. at 417, 95 S.Ct. at 2371. This purpose is not well served by a requirement that the plaintiff in every case pinpoint at the outset the employment practices that cause an observed disparity between those who appear to be comparably qualified. Such a requirement in effect permits challenges only to readily perceptible barriers; it allows subtle barriers to continue to work their discriminatory effects, and thereby thwarts the crucial national purpose that Congress sought to effectuate in Title VII. “It is abundantly clear that Title VII tolerates no discrimination, subtle or otherwise.” McDonnell Douglas Corp., supra, 411 U.S. at 801, 93 S.Ct. at 1824. Thus when unnecessary employer-created barriers have been brought into the open through adjudication of a pattern or practice disparate treatment claim, these barriers should be evaluated under the disparate impact theory, as Congress intended them to be. 2. Locating this controversy within the analytical framework. Plaintiffs, utilizing disparate treatment analysis, challenge the entirety of DEA’s employment system, and, utilizing disparate impact analysis, challenge several particular employment practices in that system (DEA’s procedures for initial grade assignments, discipline, supervisory evaluations, and promotion). In their disparate impact claims plaintiffs seek to show that these specific practices have an adverse effect on black agents, and thereby to force DEA to prove the business necessity of the challenged practices. In their disparate treatment claim plaintiffs seek to show that they are treated less well than comparably qualified white agents. Evidence of overall salary disparities, buttressed by evidence of the specific places in DEA’s employment system where discrimination occurs, supports this challenge. Whether this overall challenge will also raise disparate impact issues beyond those already raised by plaintiffs’ challenges to several specific DEA employment practices depends on the nature of DEA’s defense. See Part II-A-l-b supra. DEA has channelled the bulk of its efforts into an attempt to show that plaintiffs’ proof of disparity is either inaccurate or insignificant. DEA has bifurcated this attack into separate claims that plaintiffs’ statistical proof is deficient and that DEA’s proof in any event refuted plaintiffs’ attempt to show race-related disparities. Though made separately, both arguments go to the question whether plaintiffs have met their burden of persuasion as to the existence of race-related disparities. Subsumed in DEA’s rebuttal are several specific legal issues. DEA claims that plaintiffs’ initial case is deficient as a matter of law for two reasons. First, the regression analyses on which plaintiffs’ case is bottomed do not account for all “minimum objective qualifications” for the special agent positions at issue. See Davis v. Califano, 613 F.2d 957, 964 (D.C.Cir.1979). Second, because plaintiffs failed to produce any creditable anecdotal evidence of specific instances of discrimination, their statistical proof was required to, and failed to, meet the enhanced evidentiary threshold of showing “gross disparities” in treatment. See Hazelwood School District v. United States, 433 U.S. 299, 307-308, 97 S.Ct. 2736, 2741-2742, 53 L.Ed.2d 768 (1977). Even if plaintiffs’ proof is not insufficient as a matter of law for these reasons, DEA claims, that proof would have been found unable to stand up to DEA’s evidence had the trial court properly evaluated that evidence. One purported error DEA cites is the trial court’s misstep in placing on DEA a burden of persuasion of nondiscrimination, instead of the proper burden of coming forward with credible rebuttal evidence. See Burdine, supra, 450 U.S. at 248, 101 S.Ct. at 1089. Another claim of error involves the trial court’s purported abuse of discretion in failing to admit into evidence the alternative regression analyses that DEA offered at the remedial phase of the proceeding. DEA also claims that the trial court gave insufficient weight to its cohort analysis. Preferring to rely on a direct attack on the evidence showing race-related disparities, DEA has offered little in the way of explanation for the observed disparities. Thus DEA’s defense to both the disparate treatment and disparate impact allegations in large measure stands or falls with its arguments as to the nonexistence of the disparities that plaintiffs have sought to show. One aspect of DEA’s challenge to plaintiffs’ proof does, however, amount to an effort to provide a legitimate nondiscriminatory explanation for the observed disparity. DEA argues that plaintiffs’ statistical analysis will not support an inference of discrimination because the observed disparity was caused by a lack in the plaintiff class of a particular job qualification. Though DEA makes this argument in the context of an attack on the legal sufficiency of plaintiffs’ prima facie disparate treatment case, the claim can also be styled as a possible nondiscriminatory explanation for the disparity, and will be analyzed as such. To the extent the claim implicates a disparate impact analysis, see Part II-A-l-b supra, that analysis will also be undertaken. B. Analysis of the Liability Issues For the sake of clarity and congruence with the form of argument presented to us, we will separate our analysis along the lines suggested by the parties’ arguments. First we will examine the sufficiency of plaintiffs’ offer of proof in light of DEA’s attack on the methodology and results of plaintiffs’ statistical analyses. Then we will examine the strength of DEA’s evidence, and consider DEA’s allegations of error. Both lines of inquiry must, however, lead us toward resolution of the ultimate issue: In weighing all the evidence, did the trial court correctly conclude that plaintiffs carried their burden of persuasion as to the existence of illegal discrimination at DEA. Also, to the extent DEA’s rebuttal raises issues as to the business necessity of its employment practices, we will resolve these issues in accordance with Griggs and its progeny. 1. The sufficiency of plaintiffs’ initial case. A “general principle” of disparate treatment adjudication requires the plaintiff to “carry the initial burden of offering evidence adequate to create an inference that an employment decision was based on a discriminatory criterion illegal under the Act.” Teamsters, supra, 431 U.S. at 358, 97 S.Ct. at 1866. Because “[t]he facts necessarily will vary in Title VII cases,” McDonnell Douglas Corp., supra, 411 U.S. at 802 n. 13, 93 S.Ct. at 1824 n. 13, a specific test for the sufficiency of a plaintiff’s initial proof is not possible. Teamsters, supra, 431 U.S. at 358, 97 S.Ct. at 1866. Rather, a plaintiff’s initial proof must be measured against the more generalized functional standard that the Supreme Court has elaborated in Teamsters, supra, 431 U.S. at 358, 97 S.Ct. at 1866; Furnco Construction Corp. v. Waters, 438 U.S. 567, 577, 98 S.Ct. 2943, 2949, 57 L.Ed.2d 957 (1978), and Burdine, supra, 450 U.S. at 253-254, 101 S.Ct. at 1093-1094. These cases hold that a sufficient prima facie case is made out when the plaintiff shows a disparity in the relative position or treatment of the minority group and has eliminated “the most common nondiscriminatory reasons” for the observed disparity. Burdine, supra, 450 U.S. at 253-254, 101 S.Ct. at 1093-1094; accord Furnco, supra, 438 U.S. at 579-580, 98 S.Ct. at 2951 (A prima facie case “is simply proof of actions taken by the employer from which we infer discriminatory animus because experience has proved that in the absence of any other explanation it is more likely than not that those actions were bottomed on impermissible considerations”). The present case typifies class actions alleging a pattern or practice of dis-crimination in that the plaintiffs’ argument is largely bottomed on statistical evidence comparing the percentage and distribution of minorities in the employer’s workforce to the percentage of minorities in the labor pool from which the employer is able to draw. To be legally sufficient these statistics must show a disparity of treatment, eliminate the most common nondiscriminatory explanations of the disparity, and thus permit the inference that, absent other explanation, the disparity more likely than not resulted from illegal discrimination. Teamsters, supra, 431 U.S. at 368, 97 S.Ct. at 1871. A pattern or practice case challenges a host of employment decisions over time; in effect, it challenges an employment system. The most common nondiscriminatory explanation for a systemic disparity in treatment is a lack of qualifications among the minority group members. A plaintiff’s statistical evidence must therefore focus on eliminating this nondiscriminatory explanation by showing disparities in treatment between individuals with comparable qualifications for the positions at issue. Hazelwood School District, supra, 433 U.S. at 308 n. 13, 97 S.Ct. at 2742 n. 13; DeMedina v. Reinhardt, 686 F.2d 997, 1007 (D.C.Cir.1982) (quoting D. Baldus & J. Cole, supra, at 120). Once the plaintiffs’ analysis has focused on the proper groups for comparison, it must yield results that meet generally accepted standards of statistical significance. In other words, both the methodology and the explanatory power of the statistical analysis must be sufficient to permit an inference of discrimination. a. Methodology: minimum objective qualifications. To ensure that a plaintiff’s methodology has eliminated the common nondiscriminatory explanation of a lack of qualifications, this circuit has developed a requirement that statistical evidence of disparities account for the minimum objective qualifications for the positions at issue. DeMedina, supra, 686 F.2d at 1003; Valentino, supra, 674 F.2d at 71; Davis v. Califano, supra, 613 F.2d at 964. This requirement greatly aids evaluation of regression analyses such as those used here. Regressions seek to prove race discrimination by testing possible alternative explanations for an observed disparity between blacks and whites, and typically these alternative explanations will be particular employee traits such as prior experience or education. The minimum objective qualification approach should not, however, be read as a hard and fast rule; tests for the sufficiency of a Title VII prima facie case must not be applied in a “rigid, mechanistic, or ritualistic" way. Furnco, supra, 438 U.S. at 577, 98 S.Ct. at 2949. The minimum objective qualification approach is not a quick litmus test, but an analytic method to ensure that a plaintiff’s statistics measure disparities among comparably qualified workers, rather than disparities in qualifications. The ultimate test of sufficiency must remain that of Burdine, Teamsters, and Furnco: did the plaintiffs offer evidence “adequate to create an inference that * * * employment decisions] * * [were] based on a discriminatory criterion illegal under the Act.” Teamsters, supra, 431 U.S. at 358, 97 S.Ct. at 1866. Accord Burdine, supra, 450 U.S. at 253-254, 101 S.Ct. at 1093-1094; Furnco, supra, 438 U.S. at 577, 98 S.Ct. at 2949; Valentino, supra, 674 F.2