APA Dictionary of Psychology

n. one of the three values in a series of values that divide it into equal-sized fourths. For example, the first (or lower) quartile of a distribution is the data value below which are the lowest 25% of scores, the second quartile is the data value below which are 50% of scores, and the third (or upper) quartile is the data value below which are 75% of scores (or, conversely, above which are 25% of scores). These values provide information to researchers about the relative spread of the distribution. See Q; quantile.