What is Percentile?
Also called Percentile rank
A percentile is a position, not a proportion. A percentile of 70 means the person scored at or above 70 per cent of the norm group; it does not mean they answered 70 per cent of the questions correctly. Percentiles are also compressed in the middle of a distribution and stretched at the extremes.
The compression problem, which nobody warns readers about
Because most people cluster near the middle of a normal distribution, a small difference in raw score produces a large difference in percentile in the middle, and a large difference in raw score produces almost none at the tails.
The practical effect is that the gap between the 45th and 55th percentile is trivial in the underlying attribute, while the gap between the 95th and 99th is substantial. A shortlist that cuts at a middle percentile is slicing through the densest part of the distribution, where the instrument separates people least.
How to read one responsibly
Read the norm group first and the number second. Then read the number as a band rather than a point: with a typical reliability, the confidence interval around a mid-range percentile spans many points either side.
And never average percentiles. Percentile ranks are ordinal, so their mean has no defined meaning; convert to a standard score, average that, and convert back if a composite is needed.
Middle versus tail, on a normal scale
- 50th percentile = z 0.00; 60th percentile = z 0.25 → 10 percentile points cost a quarter of a standard deviation
- 85th percentile = z 1.04; 95th percentile = z 1.64 → the same 10 points cost 0.60 of a standard deviation
Identical movement in percentile terms, more than twice the movement in the underlying attribute. This is why a cut set at the 60th percentile is far less selective than it sounds.
Not the same as percentage correct
Percentage correct is a property of the person's answers. A percentile is a property of the person's position relative to a norm group. They coincide only by accident, and confusing them is the single most common misreading of a score report.
Related terms
More terms beginning with P
Check a selection process against the four-fifths rule
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