MyLeoNes™

Percentiles — Data, 14–17 years

A percentile tells you how a value compares with the rest of a group. It is useful when position matters more than an average.

Idea

A percentile describes a position in an ordered group. If your result is at the 80th percentile, about 80% of the results are at or below it, and about 20% are above it. It does not mean that you scored 80%.

Why

A single average can hide how people are placed across a group. Percentiles were developed to compare positions, even when the data are uneven or have a long tail. They answer questions such as “Is this waiting time unusually high?” better than an average alone.

Worked example

Seven ordered scores are 40, 55, 60, 70, 80, 90 and 95. Using the nearest-rank rule, the 75th percentile is at position ceil(0.75 × 7) = 6. The sixth score is 90, so 90 is the 75th-percentile value in this dataset.

Common trap

People often read the 80th percentile as “80% correct” or “80% of the maximum”. That is reasonable because percentages and percentiles use the same symbols. But a percentile is about rank among people or values, not about the size of the score itself.

Use

Growth charts use percentiles to show how a child’s height compares with other children of the same age and sex. Entrance tests and medical waiting-time reports can use them too. The comparison group and its date matter, because a percentile is always relative to a dataset.

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