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By Kim Glenchur
Percentile ranks indicates the percentage of students in the representative norm or sample group who scored below your child's score. A 60th percentile indicates that the child scored better than 60% of the population norm. Percentile ranks are a numerical ordering of test scores, from 1 to 99. This ranking method, however, does not provide information on the intervals between percentile values. For example, a few points difference in the score around the middle could dramatically alter a child's percentile ranking. Conversely, a few points difference at the very low or very high end may not result in any change in the child's percentile rankings at all.
Mean (often symbolized by ì; known in the dictionary as mu) is the average of the test scores of the norm, calculated by adding the values of the test scores and then dividing this sum by the number of tests taken. The best psychological tests should have been developed using a large number of individuals that would be truly representative of a certain target population. For example, in a general ability or achievement test, an average result really would be the average score of the general population having the same characteristics as the test-taker.
If the test scores of the large population sample were lined up in numerical order, most of the scores would be near the mean. Some scores would lie quite far from the mean. Standard deviation or SD (often symbolized by ó, known in the dictionary as sigma) is the rough equivalent of the average distance of the test scores from the mean. (A basic statistics text should provide information on the exacting calculation of standard deviation.) When the scores have a normal distribution, about 68% of all the test scores lie within one standard deviation from the mean (either below or above). About 95% of all the test scores lie within two standard deviations from the mean. About 99.7% of all the test scores lie within three standard deviations.
Significant or meaningful differences are two standard deviations away from the mean. For example, if one standard deviation is a span of 15 points away from the mean on a particular test, then two standard deviations equals 30 points away from the mean. On IQ tests with SD = 15 and mean = 100, an IQ at or below 70 is "mentally retarded", and an IQ at or above 130 is "gifted."
Tests and assessments have limits. A quick test is unlikely to accurately diagnose the cause of a multi-dimensional learning disability for example. Be forewarned that some test administrators "ignore measurement limitations, are unfamiliar with research literature, or lack training and experience with children."2
Reprinted with permission from Kim Glenchur. All rights reserved.
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