The problem for respondents is that a prima facie case of disparate-impact liability—essentially, a threshold showing of a significant statistical disparity, ... and nothing more—is far from a strong basis in evidence that the City would have been liable under Title VII had it certified the results. That is because the City could be liable for disparate-impact discrimination only if the examinations were not job related and consistent with business necessity, or if there existed an equally valid, less-discriminatory alternative that served the City’s needs but that the City refused to adopt. ... We conclude there is no strong basis in evidence to establish that the test was deficient in either of these respects.
Let's do some boolean algebra:
A = job related.
~A = not job related.
B = consistent with business necessity.
~B = not consistent with business necessity.
(A & B) = job related and consistent with business necessity.
~(A & B) = "not job related and consistent with business necessity"
~A | ~B = not job related or not consistent with business necessity.
C = equally valid, less-discriminatory alternative
Those of you with training in logic know that, per De Morgan's Law, ~(A & B) = ~A | ~B. Thus, the formula for a disparate impact judgement appears to be ~A | ~B | C: one or more of not being job related, not being consistent with business necessity, and the existance of an alternative.
Question: why "job related"? This was always considered an easier standard to meet than "business necessity", which is why the 1991 Civil Rights Act enshrined "business necessity" as the standard that defendants must meet to justify disparate impact.