Page 35 - teachers.PDF

Percentile scores (ranks)
Percentile scores tell you the percent of students in the norming sample whose scores were at or lower than a given score. Percentile scores are among the most commonly reported scores and are best used to describe a student's standing in relation to the norming group at the time of testing. For example, if a student's score is in the 80th percentile, then that student scored equal to or higher than 80% of the students who took the test when the test was normed.
Note that although percentile scores are reported in increments of one hundredths, they are not completely accurate. When you use percentiles, you should pay attention to the confidence bands that the test publisher provides.
Confidence bands represent the range of scores in which a student's true score is likely to fall. For example, although a student's score on a particular test may be at the 86th percentile, it is likely that if the student took the same test on a different day, the new score would vary slightly. Accounting for random variations, that student's true achievement may fallsomewherewithinarangeofscores,forexample,betweenthe81st and89thpercentiles.
Percentile units are used to report an individual student's score; they should not be averaged to describe groups. Percentile units cannot be subtracted to compute gains because differences in percentile scores are not constant across the entire scale. For example, getting an additional two items correct can greatly increase a percentile rank for an average student. Yet the score increase from the same two items may not result in any percentile change for students of very above average achievement. Score gains increase percentile ranks more in the middle of the range than toward the extremes. (See Figure 3.)
How are percentile scores distributed?
Figure 3 shows how percentile scores are distributed when raw scores are distributed normally. The y axis shows the proportion of students and the x axis shows the percentile score. Vertical lines have been drawn to indicate each standard deviation unit. Note that the percentile scores are not evenly distributed on the x-axis. If they were evenly distributed, then the proportions graphed on the y-axis would all be the same; each proportion would be 1%!.
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Rudner, L. and W. Schafer (2002) What Teachers Need to Know About Assessment. Washington, DC: National Education Association.
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