The percentile rank of a score is the percentage of scores in its frequency distribution

Frequency distribution

In statistics, a frequency distribution is an arrangement of the values that one or more variables take in a sample. Each entry in the table contains the frequency or count of the occurrences of values within a particular group or interval, and in this way, the table summarizes the distribution of...

 that are the same or lower than it. For example, a test score that is greater than 75% of the scores of people taking the test is said to be at the 75th percentile

Percentile

In statistics, a percentile is the value of a variable below which a certain percent of observations fall. For example, the 20th percentile is the value below which 20 percent of the observations may be found...

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Percentile ranks are commonly used to clarify the interpretation of scores on standardized tests. For the test theory

Test theory

In experimental physics, a test theory tells experimenters how to perform particular comparisons between specific theories or classes of theory....

, the percentile rank of a raw score is interpreted as the percentages of examinees in the norm group who scored below the score of interest.