Suppose you have N numbers.
Find the lower and upper quartiles QL and QU. These are the numbers at positons N/4 and 3N/4.
(If N/4 and 3N/4 are not whole numbers, the quartiles are determined by linear interpolation between the two adjacent numbers. eg if N=21, N/4=5.25 so QL is 0.75(5th)+0.25(6th); 3N/4=15.75 so QU is 0.25(15th)+0.75(16th).)
The interquartile range is QU-QL.
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