The terms "standard score" and "percentile rank" are often used together to describe tests_scores on intelligence and other standardized tests. Both refer to a calculation comparing an individual's score to the performance of all participants on the same test, usually across a specified age range.
A standard score is a type of score that has been normed, meaning it has been calibrated to a mean of 100 and a standard deviation of 15. The score expresses how many standard deviations a particular score is above or below the mean. For example, a score of 100 on the Wechsler Intelligence Scale for Children (WISC) would indicate that the child scored at the mean, or had no deviation from the mean. Scores above and below the mean are represented by standard scores that indicate how many standard deviations each score is above (positive standard scores) or below (negative standard scores) the mean.
percentile rank, on the other hand, is a measure of an individual's absolute strength or achievement relative to others on the same test. It tells us where a score falls in relation to all other scores on the test, not just those of the same age group. For example, a percentile rank of 50% on an intelligence test indicates that the child scored as well as or better than 50% of their peers who took the test. Scores at the 90th percentile, for instance, would mean that the child performed better than 90% of their age group, which is considered average.
To properly compare standard scores and percentile ranks, one must consider the reference group against which the scores are being compared. Standard scores are often used to compare an individual's raw score from a test to the mean and standard deviation of a larger group of participants. Percentile ranks, on the other hand, are used to compare an individual's score to all other participants on the same test, providing a broader context for understanding an individual's performance.
