Technical reports on test scores typically distinguish differences that are “significant” from those that are not. But this distinction is not always useful for policy purposes and is frequently misunderstood by policymakers. To a technical expert, a score difference can be miniscule but still “significant” if it can be reproduced 95 percent of the time when a comparison is repeated. But miniscule score differences should be of little interest to policymakers. In general, social scientists consider an intervention to be worthwhile if it improves a median subject’s performance enough to be superior to the performance of about 57 percent or more of all subjects prior to the intervention. Such an intervention should be considered “significant” for policy purposes, but, to avoid confusion, we avoid the term “significant” altogether. Instead, for PISA, we consider countries’ (or social class groups’) average scores to be “about the same” if they are less than 8 test scale points different (even if this small difference would be repeated in 95 of 100 test administrations), to be “better” or “worse” if they are at least 8 but less than 18 scale points different, and “substantially better” or “substantially worse” if they differ by 18 scale points or more. Eighteen scale points in most cases is approximately equivalent to the difference social scientists generally consider to be the minimum result of a worthwhile intervention (an effect size of about standard deviations). The TIMSS scale is slightly different from the PISA scale; for TIMSS, the cut points used in this report are 7 and 17 rather than 8 and 18.
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