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 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 add’l percentiles for higher scores) using that basic formula for students included in the norming sample (see https://help.easycbm.com/how-were-norms-for-the-benchmark-and-progress-monitoring-assessments-calculated/ for more information on the norming process). These are the percentiles you see reported in student reports and in the Detailed Percentiles Table or the Progress Monitoring Scoring Guidelines available on the Reports page after logging into your account.

So, 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.