## Getting to know the range, interquartile range, and standard deviation

The three most important measures of dispersion are defined as follows:-
The

*range* -
The

*interquartile range* -
The

*standard deviation**mean score*

## Working out which measure of dispersion to use

You determine the most appropriate measure of dispersion as follows, depending on the nature of your data:-
**Data measured at the nominal level:**Because all three measures of dispersion require data to be ranked or summed, none of them are appropriate for data measured at the nominal level. -
**Data measured at the ordinal level:**The range and interquartile range are appropriate. The interquartile range is usually preferable, as it is more informative than the range. -
**Data measured at the interval/ratio level:**All three measures of dispersion we have examined are appropriate. The standard deviation is usually preferable. However, the standard deviation (or variance) isn’t appropriate when there are extreme scores and/or skewness in your data set. In this situation the interquartile range is usually preferable.