APA Dictionary of Psychology

a graphical display of the central value, variance, and extreme values in a data set. A rectangle (box) is drawn along the vertical y-axis of the plot, which shows the range of data values. The length of the box indicates the middle 50% of scores and its two ends indicate the upper and lower hinges. Lines (whiskers) extending outward from the box denote variation in the upper and lower 25% of scores, while a separate line within the box indicates the score that falls in the very middle of the set (i.e., the median). Stars or other single points indicate extreme scores. A box-and-whisker plot is useful in exploratory data analysis for indicating whether a distribution is skewed and whether the data set includes any outliers; it can also be used to compare data sets when large numbers of observations are involved. Also called boxplot.