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EXAMINING THE “PLATEAU EFFECT”: New Report Finds Phenomenon is Less Prevalent Than Previously Believed

“Raising the stakes attached to test results can lead to a substantial increase in performance, even on tests that have already been in place for four or five years.”

According to the “plateau effect”, student test scores skyrocket during the first few years after a new test is introduced, followed by a leveling of test scores after all of the “easy” ways of making gains have been exhausted. Within the education community, some observers used the plateau effect to explain the early increases seen after the implementation of the No Child Left Behind Act (NCLB). A new report from the Center on Education Policy (CEP) shows the phenomenon is far less prevalent in student testing than previously believed. However, the report, State Test Score Trends Through 2007–08, Part 2: Is There a Plateau Effect in Test Scores?, also finds a significant upswing in student test scores after the enactment of NCLB.

Under NCLB, schools have until School Year 2013–2014 to help all students reach adequate yearly progress (AYP), a state-defined level of proficiency in reading and math. The existence of the plateau effect could lend proof to the argument that, since the enactment of NCLB, initial gains in test scores are “low-hanging fruit”—meaning that once the majority of students have reached the required proficiency level, it is extremely difficult to bump up the remaining students.

Although the education stakeholder community commonly refers to the plateau effect, the report claims it is “based on a small number of states and on test data from the 1980s and 1990s.” The CEP study analyzes more recent data from a wider draw of states. The raw data was pulled from the CEP database of test results in reading and mathematics from elementary, middle, and high school students from as far back as 1999. The study does not include data from all fifty states and the District of Columbia; it only includes data for those with at least six years of data available. Otherwise, the trend lines might not have been long enough to allow for the possibility of a plateau.

The report’s authors conclude that although a plateau effect is apparent in some states, it is not pervasive across the nation. Overall, the only common pattern is an upward trajectory of student scores. The study uses fifty-five trend lines to examine performance advancement. Of the trend lines studied, fifteen exhibit a plateau effect, twenty one show steady increases, and nineteen illustrate a zigzag pattern, meaning student performance increased and decreased multiple times.

For more than one third of the trend lines analyzed, the report finds that the largest jump in percentages proficient occurred between 2003 and 2004. Because the first full school year of testing under NCLB occurred between 2002 and 2003 and the second between 2003 and 2004, the study cites a possible “NCLB effect.” The report’s authors are careful to point out that it is “somewhat difficult to establish clear causation because of older policies being implemented at the state and local levels at the same time.”

examining the

The report also examines trends according to the number of years a test had been in place, and finds large gains occur soon after a new test is introduced. “Raising the stakes attached to test results can lead to a substantial increase in performance, even on tests that have already been in place for four or five years,” it reads. As indicated in the chart to the right, CEP found a drop-off in increases during the later years but attributed it to the limited number of trend lines available with nine to ten years of data.


The complete report is available at


No Child Left Behind

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