The most basic measure of variability is the range.

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Since the range measures variability by looking at the end-points of a distribution, it is extremely sensitive to the presence of outliers in the dataset. An alternative measure of variability that is much less sensitive to outliers is the interquartile range.
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Remember that quartiles are measures of location which divide a distribution into four equal parts, similar to how the median divides a distribution into two equal parts. The interquartile range thus measures how spread out the middle 50% of the data is. This means that the interquartile range is completely unaffected by the values of the smallest and largest 25% of the scores.

Often, the range and interquartile range are combined with the median to form a so-called five-number summary.
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