From TheBestLinks.com
In descriptive statistics, the interquartile range (IQR) is the difference between the third and first quartiles. The interquartile range is a more stable statistic than the range, and is usually preferred to that statistic.
Since 25% of the data are less than or equal to the first quartile and 25% are greater than or equal to the third quartile, the difference is the length of an interval that includes about half of the data. This difference should be measured in the same units as the data.
Example
This uses the data from the quartile page.
i x[i]
1 102
2 105
------------ the first quartile, Q1 = 105 + .25(106-105) = 105.25
3 106
4 109
------------ the second quartile, Q2 or median = 109 + .5(110-109) = 109.5
5 110
6 112
------------ the third quartile, Q3 = 112 + .75(115-112) = 114.25
7 115
8 118
From this table, the interquartile range is 114.25 - 105.25 = 9
See also: statistical dispersion, mean
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