As college attendance became more common, the proportion of high-school graduates electing to take admissions tests rose, and many of those newly added to the rolls were lower-scoring students. This was studied in considerable detail by the College Entrance Examination Board in the 1970s, and the research showed clearly that a sizable share of the drop in SAT scores was the result of this compositional change. Had the characteristics of the test-taking group remained constant, the decline would have been much smaller.
