Help Center : District Edition

How do you calculate/interpret percentiles? Why does a perfect score not always equate to the 99th percentile?

The formula for calculating percentiles is given as:  B / T * 100 = percentile

T = Total number of test-takers
B = Number of test-takers whose score are at/below a given score

easyCBM tests are largely norm-referenced fundamental skills assessments. Thus, a student’s performance is compared to national norms––scores based on a large nationally-representative group of same-grade students on the same assessment. Our norm-referenced scores are reported in raw totals, but also, in percentiles (1-100). So, for these assessments, think of baby growth charts, where children’s weight and height are compared to one another. Just like those charts, a student’s percentile rank for norm-referenced assessments compares their performance to other grade-level peers.

For example, a perfect 20/20 on the Grade 3 Winter Benchmark in Vocabulary is represented by the 87th percentile in reporting because, in the norming sample, 87% of students scored below 20/20 on the Grade 3 Winter Benchmark in Vocabulary. Or, in other words, 13% of students in the norming sample scored 20/20 on that measure.

For most easyCBM test forms we calculate a percentile for every possible score (within reason…i.e., for oral/passage reading fluency, once we get to 99th percentile for a given raw score there is no need to keep calculating percentiles for higher scores) using that basic formula for students included in the norming sample.

Last Updated: July 21st, 2022
Filed under: FAQ