From 1999 to 2006, the Boston Police Department (BPD) (defendant) tested officers’ and applicants’ hair for traces of drugs. The test was negative for over 99 percent of white people tested and over 98 percent of black people tested. The hair drug test was reliable but not 100 percent accurate, because it could present a false positive due to failure to distinguish between a person’s consumption of drugs and a person being in an environment where drugs were present. In 2003, Dr. Kidwell described in an affidavit to the BPD an alternative drug testing method. Under Kidwell’s method, if a person failed the hair test, there would be a follow-up urinalysis test. The BPD continued to use the hair test. The BPD did use urinalysis tests for officers that were suspended for drug use. Jones and other black individuals (plaintiffs) took the BPD’s hair drug test and tested positive. The plaintiffs were either fired from the BPD or not hired on account of the test results. The plaintiffs brought suit against the BPD, alleging disparate impact on the ground that the hair of black people is more likely to absorb environmental drugs. The result, according to the plaintiffs, was a disproportionate number of false positives for black police officers. The district court granted summary judgment to the BPD. The plaintiffs appealed.