Different AP classes have different difficulty rates so it wouldn’t make sense to compare AP Government results to AP Physics. Also, different students take different classes, so you have a self-selected group taking AP Latin or Calculus BC. He gives an example of AP Biology and AP Psych but I would bet that the difficulty levels of those tests vary and the students taking the class might be different sets of kids.
And different schools have vastly different populations. He seems to assume that it is the same group of bright kids across the board taking APs. I was talking to an AP Government teacher from an inner-city school in Michigan this summer, and he has never had a student pass the test. I am not vain enough to think the difference in our results is due to me.
Many schools now encourage students to try AP classes, even if they’re not likely to do well enough on the test to earn college credit. Other schools reserve AP classes for the very best students. I’d hate to see teachers have an incentive to exclude good-but-not-great students from taking the AP challenge.