4.8 Box and Whiskers Diagram
A box and whiskers diagram or boxplot as it is also called, is a dispersion measurement and is especially useful when comparing values from several groups or from several points in time. It shows the larges and smallest values, the middle value – median, and the quartiles which make up the ends of the “box” which contain 50 % of the data.
We will look at an example:
A group of people have the following ages, written in order.
Lower Quartile: the limit for the first 25 % of the values = 17
Upper Quartile: the limit for the first 75 % of the values = 25
Interquartile Range: The upper quartile minus the lower quartile =
25 – 17 = 8
The boxplot looks like this when we draw it on the number line:
We summarise:
 Least and greatest value
 The median, the middle value
 Lower Quartile (25th percentile)
 Upper Quartile (75th percentile)
