[T]he percentage of ALL graduating seniors, including those who never got near an AP course, who had at least one score of 3 or above on at least one AP test sometime in high school.

Challenge Index’s unabridged methodology
By Jay Mathews 5/23/2011

AND SEE:
AP pass rates
55.6% I.H.S. pass at least 1 AP exam by graduation
Why the AP pass rate matters
What is the Equity and Excellence score?
College readiness scores (US News)