The mean and median are considered measures of center as they represent the average of a data set. The average basically being a point that collectively speaks for all of the data. For example, if you have a group of basketball players whose heights range from 5'11" to 6'9", then there is no single height to report; however, we can compute the average to get a basic idea of the single height. The interquartile range (IQR) is a measure of variability or spread of the data. The higher the IQR, the more spread out the data is. Recall that 50% of the data is represented by the IQR and that IQR = Q3 - Q1 where Q1 and Q3 are the first and third quartiles respectively. So because the IQR is a measure of spread, it is not considered a center point.
