My students took more than one version of the same exam on the regularly scheduled testing date. Why does this occur, and how does College Board decide which version of the AP Exam my students will receive?

When necessary, we administer alternate exam versions to small groups of students in the United States on the regularly scheduled testing date. This is to ensure that scores for different exam versions (worldwide) can be compared in a statistically valid way. However, this does also mean that some or all students in one school may take a different version of the exam than the most commonly administered version(s).