IANA lawyer, but I deal with this stuff. My understanding is that case sites are largely not relevant, because each case is handled on its own peculiar situation.
That is, what may be disparate impact in one situation may NOT be disparate impact in another situation. And what may be a business necessity (the technical term is “bona fide occupational qualification” or BFOQ) in one situation may not be in another. There is a huge body of case law, and the burden of proof rests with the employer to prove something is a BFOQ. Generally speaking, the courts and the EEOC have interpreted “business necessity” fairly narrowly.
Preferences of employer, coworkers, or clients are irrelevant and do not constitute BFOQs.
In short, an educational requirement such as college degree may not have illegal discriminatory impact for a job such as high-tech medical device sales, but might for a job such as shoe salespeson.
Griggs vs Duke Power was the most significant EEO case ever, and set down the principles applied in other cases. You might look at McDonnell Douglas vs Green (1973) (411 US 567) which describes the legal standard for circumstantial evidence of adverse impact. There’s also Watson vs Fort Worth Bank & Trust (1988) (108 S. Ct. 299) dealing with employment interviews and subjective vs objective assessment procedures.
For those reading this who may want explanation: under U.S. law, a practice can be discriminatory in and of itself (such as, “We don’t hire Koreans.”) – this is called “disparate treatment” and is blatantly illegal. However, a practice that appears to be non-discriminatory on the surface could still have the effect of discrimination; this is called “disparate impact.” Thus, the law prohibits both open and deliberate discrimination (disparate treatment) and also practice that seem fair in form but are discriminatory in operation (disparate impact.)
EXAMPLE: A company has a policy of only hiring people for a certain job who live within a ten minute drive of the office. There are very very few blacks in that area, and so the effect of the hiring practice is disciminatory – “disparate impact” – even though the practice itself is not discriminatory per se.
The company defense might be that this is, in fact, an essential job requirement,a bona fide occupational qualification (BFOQ): that the person must be able to get to the office with ten minutes in an emergency. BFOQs are exempt from the discrimination laws (Obvious example: if you want to hire an actor to play the role of George Washington in a movie, you can refuse to audition blacks, Asians, females, etc … and this is not viewed as illegal discrimination, since having a white male actor for the role is an essential job requirement.)
Another exemption from the disparate impact type of discrimation includes seniority-based programs. Males tend to have more seniority than females because many females take time off to raise a family; thus, a seniority-based program is exempt from the disparate impact test. (A seniority-based system that involved disaprate treatment or an intention to discriminate, however, would be illegal.)