A ranking among a population, in which a member's percentile ranking represents the percentage of the population ranked beneath the member. The ranking can be from 0 to 99 (one can never rank above the entire population, which includes oneself).

While common usage is frequently to say "I scored in the footh percentile," it is more correct to say, "I scored above the footh percentile." A percentile, like a quartile or a decile, is a point, not a range. The standard abbreviation for percentile is %ile.

Uses:

Standardized Tests
In tests such as the SAT, GRE, or AFOQT, test-takers receive a raw score, which shows how well they performed on the test. Each participant also receives a percentile score, showing how well they did compared to the rest of the test takers in that group. In this fashion, people who took different tests can, in theory, be objectively compared.
Alyssa P. Hacker and Ben Bitdiddle each take the SAT, in different years. Alyssa scores a 1400, which is 98th %ile. Ben, who takes the new, politically correct SATs, scores a 1460, quantitatively higher than Alyssa. However, he is only the 93rd %ile - Alyssa ranks qualitatively higher.
95/5 computation
For bandwidth billing, usage slots are ranked by consumption of bandwidth.
To find a percentile rank all of the numbers in a given set from largest to smallest. Then, go along the list of numbers to the number that represents the percent you are looking for. For example:

In a set of 100 numbers, the number that is 10% of the way through the set is the tenth number. The value of this number is the tenth percentile.

Since the number 50% of the way through the set is the middle number and, since the median of a set is the value of the middle number, the 50th percentile of a set is equal to the median.

Since the 50th percentile represents a percentile divisible by ten it is what is called a decile. Since it is also divisible by 25 it is also a quartile.

Percentiles are a statistical tool used to examine the spread of a set.

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