A percentile is best described as a comparison score. It’s a common term in all kinds of testing of data, but many will be most familiar with percentiles as they relate to standardized testing in K-12 schools. Unlike percentages, where you are given a percent number that is related only to your performance, percentiles relate the number to the performance usually of 100 similar students.

It’s easier to understand this when you can look at comparisons of percentages and percentiles. For the sake of simplicity, consider a test with 100 problems on it, each problem worth 1% of the test. If you get 80 problems correct, you get 80% on the test.

80% refers to the number you got correct, not to your score in reference to anyone else’s performance. Some teachers grade on a curve, and award the highest grade not based on actual percentage, but on highest test score. So if your 80% was the highest score in the class, you’d earn an A. More standard is assigning As to 90% and up, Bs to 80-89%, Cs to 70-79% and so on.

Especially when testing is standardized, it is meant to serve a diverse group of people and accurately gauge not only individual performance but comparative performance. If we look at the same test again, your score in percentiles would be based on the number of students who scored below you on the test. So if you did get the top grade, and there were 100 students taking the class, you would score in the 99th percentile. This means you had a score better than 99% of the people taking the test. This is far different than your base 80% score.

When looking at a data set, percentiles can help better gauge the middle or median performance of students, or of any type of data. Many students will cluster into the median area, earning percentiles anywhere from 25 to 75. A few students will far surpass this, with percentiles in the 90s range. Even when the student population or data set being tested is large, average and median scores are computed into expected results, and can gauge the average desired performance of a student. Percentiles can show us how most people are performing, as well as how each individual student is performing.

Percentiles can further show if performance in certain areas is poor. If every student taking a test misses the same question, or if most of the average students taking it miss it, this may suggest a few things. The students may not understand the question, or they may need more instruction in this area. In this way percentiles can also reflect how well prepared a test is. With increasingly more standardized tests in the academic setting, percentiles have a way of weeding out bad questions, when a large number of students don’t answer a question correctly.

Percentiles can be used to check testing in population groups. Say for example, a whole high school in an urban neighborhood scores well below average. Even if a couple of people score well (called outliers), the school is evidencing a couple of things. Either the children are not prepared properly to take a test, or the test is not understood by the students because of cultural or language barriers. By considering percentiles as well as percentage scores, schools can better address the needs of their students in a holistic fashion.

It bears repeating that percentiles are a comparison score. The number of a percentile represents how well or how poorly you did as compared to other students. It does not represent the number of questions you answered correctly. If you score in the 70th percentile, you scored better than 70 out of 100 people who took the test. If you score in the 50th, read this as better than 50 people who took the test.