Percentile rank vs. nth percentile

In this course we look at simple CLAST-style questions that refer to the definition of percentile rank: the percentile rank of a score in a distribution is the percent of the population that achieved less than the given score, or, more formally, "the percentage of scores in a specified distribution that fall below the point at which a given score lies. "

Please aware of the fact that there is another term in statistics that sounds similar but has a different meaning. This other term is "nth percentile." A formal definition of "nth percentile" is "the smallest value in the set with the property that n% of the data values are less than or equal to it."

A statement such as "a score of 600 has a percentile rank of 80" has a different meaning from a statement such as "the 80th percentile is 600."

The first of these two statements says that 80% of the population had scores that were less than 600, while the second statement says that 80 percent of the population had scores of 600 or less. Statements involving nth percentiles, which are not employed in this course, are typically used for defining the cutoffs when the scores have been arranged numerically into 100 equal intervals.

The calculations in this course involve percentile rank, not (nth) percentiles.