Key Formula
Percentile Rank = (Number of values below x / Total values) × 100
Common Mistakes
- Confusing percentile with percentage — they measure fundamentally different things and are not interchangeable.
- Assuming percentiles are evenly spaced — the gap between the 90th and 95th percentile is often much larger in raw score terms than between the 50th and 55th, especially in skewed distributions.
Frequently Asked Questions
What are quartiles and how do they relate to percentiles?
Quartiles are simply the 25th, 50th, and 75th percentiles — they divide a data set into four equal-sized groups and are commonly used to describe spread.
Are percentiles affected by outliers?
Less than the mean is, but extreme outliers can still shift percentile rankings somewhat, particularly in smaller data sets where each individual value carries more relative weight.
