A Box Plot is a graphical view of a data set which involves a center box containing 50% of the data and “whiskers” which each represent 25% of the data. It divides the distribution of a data set into four portions: the lower “whisker” contains the first quartile or 25% of the data, the lower segment of the box contains the second quartile, above the median line to the top of the box is the third quartile and the upper “whisker” represents the 4th quartile of data.
The midpoint of the box represents the median of the data set, and its position indicates if there is any skew to the data.. The endpoints of the top and bottom whiskers represent the largest and smallest data points, respectively. Outliers, or data that does not fit the predominant distribution, are represented by asterisks at the top and bottom of the box plot. This graph is most useful when comparing two or more strata or data sets.