Is There a Plateau Effect in Test Scores?

Many in the research, policy, and media worlds have taken for granted the existence of a phenomenon known as the “plateau effect,” wherein test scores rise in the early years of a test-based accountability system and then level off. The theory holds that the first few years of score gains, in which teachers and students are rapidly adjusting to the new test, are “low hanging fruit,” and that scores plateau in later years once the “easy” ways of making gains have been exhausted. But the most comprehensive study of the plateau effect to date, released today by the Center on Education Policy (CEP), calls this phenomenon into question.

Drawing from its database of reading and math test results from all 50 states going back as far as 1999, CEP researchers looked for evidence of a plateau effect in 55 trend lines from 16 states with six to ten years of consistent test data. The study revealed several main findings:
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In the current testing context, one cannot assume the existence of a plateau effect when trying to predict state test score trends.
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The largest gains did not consistently show up in the early years of a testing program.
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A clear upswing in test results was apparent after the enactment of the No Child Left Behind Act (NCLB).
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In the three states with longer trend lines, gains generally did level off after nine or ten years, but the data were too limited to know whether this is a consistent pattern in state test performance.

The full report, State Test Score Trends Through 2008: Is There a Plateau Effect in Test Scores?, is the second in a series of 2009 CEP reports analyzing student achievement trends.