How Is a Percentile Rank Different from a Standard Score?
What exactly does a percentile rank mean? Is the 50th percentile a “bad score?” Does it mean that the student knew 50% of the material? Does a standard score of 100 mean he knew all the material?
Both standard scores and percentile ranks relate the individual’s result to those of all students who took the same test. Some assessments assign the average result for the test a standard score of 100, and assign higher and lower scores accordingly. The Wechsler Intelligence Scale for Children is a good example of this kind of test. A percentile rank of 50 means the student performed better than 50% of the children her age who took the same test – also an average result. So a student who scores 100 on the WISC is at the 50th percentile relative to same aged peers.
For most standardized assessments, ‘average’ ranges from the 25th to the 75th percentile. Fifty percent of the population will fall in the average range, whether we are talking about height, reading scores, or intelligence. In the case of intelligence tests, standard scores are reported as an “IQ” score. A standard score of 100 on a standardized test of math or reading would also be average.
As a standard score gives a position in relationship to a reference group, the score by itself is meaningless without knowing the position of the student when compared to all students who took the same test. A standard score of 90, the beginning of the average range, corresponds to a percentile rank of 25. A standard score of 110, the uppermost end of average, has a percentile range of 75. So a child at the 30th percentile on a test of reading or math is performing within the region of what would be considered “average.”
With percentile ranks and standard scores, the comparison group matters. High school juniors and seniors who want to go to college take a well-known standardized test, the American College Test (ACT). The ACT Board reports that for 2013 a score of 22 is at the 50th percentile for Ohio students. Students who received a score of 18 were at the 25th percentile, and those who obtained a score of 25 were at the 75th percentile. Knowing the comparison group and the percentile rank puts the score in perspective.
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