Divide a set of data into two groups (high and low) of equal size at the statistical median if there is an even number of data points,
or two groups consisting of points on either side of the statistical
median itself plus the statistical median
if there is an odd number of data points. Find the
statistical medians of the low and high groups,
denoting these first and third quartiles by 
and 
. The interquartile range is then defined by
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