Concept
Read a percentile as a rank, not a score
The 90th percentile is a position in a crowd, not ninety correct answers out of a hundred.
When it fits
- A test, service report or benchmark presents a percentile.
When to avoid it
- Sample size, ties and calculation conventions affect exact values; a historical percentile is not a guarantee.
The idea
Identify the reference group, the measured quantity and the direction that matters. A percentile locates a value within ordered observations. A high percentile can be desirable for a score and undesirable for a waiting time.
An example
A reported 90th-percentile waiting time is a tail threshold, not the maximum wait or a promise to every future customer.
Check your result
You can translate the percentile into a sentence about the distribution it summarizes.
Keep this limit in mind
- Sample size, ties and calculation conventions affect exact values; a historical percentile is not a guarantee.
Connected ideas
Useful withPut variation beside the average
Evidence and sources
Supports
A percentile describes a position in an ordered distribution, not the percentage of a task that someone completed correctly.
Ties, small samples and software conventions can affect exact percentile calculations.
Introductory Statistics 2e, 2.3: Measures of the Location of the Data · Percentile definitions and interpretation