[nepstein]

True - though this means only that the signal (outperformance by a group that is the target of discrimination) might not be present (if discrimination continues past initial selection as you point out).

However, the test may still useful to help confirm bias. If outperformance is observed, you can infer one of 3 things is true:

1) there is bias at initial selection but not after (or at least reduced bias)

2) members of the outperforming group are simply stronger performers (different but still interesting)

3) there is no bias at selection but there are affirmative action effects after the initial selection (not obvious why this would be the case)