Why have some AP subjects' score distributions changed recently?

AP’s standards are anchored in evidence. We periodically conduct research on each AP subject around the performance of AP students who use their AP credit to place out of introductory college courses. In this way, on a rotating basis, each AP subject's standards are verified every 5–10 years. This research results in one of three outcomes: the data support a larger proportion of AP students receiving credit-qualifying scores; the data confirm and maintain the same standards as in the past; or the data require reducing the percentage of students receiving particular scores within the 1–5 range.