Summary of Measures of Spread (Variability)



The standard deviation is by far the most widely used measure of spread. It takes every score into account, has extremely useful properties when used with a normal distribution, and is tractable mathematically and, therefore, it appears in many formulas in inferential statistics. The standard deviation is not a good measure of spread in highly-skewed distributions and should be supplemented in those cases by the semi-interquartile range.

The range is a useful statistic, but it cannot stand alone as a measure of spread since it takes into account only two scores.

The semi-interquartile range is rarely used as a measure of spread, in part because it is not very mathematically tractable. However, it is influenced less by extreme scores than the standard deviation, is less subject to sampling fluctuations in highly-skewed distributions, and has a good intuitive meaning. It should be used to supplement the standard deviation in most cases.