To make a box-and-whisker plot, mark the minimum, first quartile, median, third quartile and maximum of your data set above an evenly spaced number line, and draw boxes from the median to the first and third quartile and whiskers from the first quartile to the minimum and from the third quartile to the maximum. A box-and-whisker plot provides a visual representation or five-number summary of your data. Statistics students often use this type of plot.
A five-number summary is a concise explanation of the distribution of your observations. The median divides your data into two halves, and the first and third quartiles are the medians of the lower and upper halves, respectively. The maximum and minimum are the lowest and highest numbers in your data set, excluding outliers.
In a box-and-whisker plot, whiskers do not extend to outliers. Instead, a dot or an asterisk above the number line marks whiskers. To determine if a data point is an outlier, find the interquartile range, or IQR. The IQR is the distance between the first and third quartiles. If a data point is above the third quartile or below the first quartile by more than one and a half times the IQR, it is an outlier.