A percentile tells you the value below which a given percentage of data falls. If a test score is at the 90th percentile, 90% of scores are below it — it’s about relative standing, not the raw score or a percent correct.
Quartiles
Quartiles are just common percentiles: the 25th (Q1), 50th (the median), and 75th (Q3) divide data into quarters. The gap between Q1 and Q3 — the interquartile range — describes the middle 50% of the data.
Finding a percentile
Sort the data, then locate the position for the percentile you want. A simple approach: position ≈ (percentile/100) × (n + 1), counting into the sorted list and interpolating if it lands between two values. Percentile rank goes the other way — it tells you what percentage of values fall below a particular data point.
Frequently asked questions
What does 90th percentile mean? 90% of the data falls below that value.
Is a percentile the same as percent? No — it’s relative standing in a dataset, not percent correct.
What are quartiles? The 25th, 50th, and 75th percentiles.
Percentiles are ideal for comparing across different scales — a child’s height “at the 60th percentile” means more than the raw centimeters, because it places them against peers. That’s why growth charts and standardized tests report results this way.