Why are the percentile ranks for students and schools different?

Document created by Community User on Jan 9, 2017Last modified by Community User on Jun 6, 2017
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When I look at the percentile rank for a student with a particular RIT score, the percentile is much different than the percentile rank for a school or a grade with the same or a similar RIT score. Why?

The norms for schools are calculated from sets of aggregated data. Instead of comparing individual students to other individual students, we are comparing groups of students. When you gather the average RIT score for many different groups of students, the averages of these groups will vary less than the scores for individual students.

Therefore, the standard deviation for schools is smaller than the standard deviation for students. When the standard deviation is smaller, this results in a different distribution of percentiles.
The below graph shows two different distributions, one with a greater standard deviation (like the student norms) and another with a smaller standard deviation (like the school norms). It illustrates how the same RIT can correlate to different percentiles:

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