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In this case, research has shown that kids who took AP courses in HS do better in college. He concluded that it means AP courses help one to do better! I will leave it to you dear readers to explain the alternative explanation...
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<p>If I understood NMD correctly, the correlation might not be between the AP course and college performance but between AP-taking student and college performance. It's a little bit like the argument of value-added education at HYPS vs. at a less selective school.</p>