Given:
61,65,65,66,72,75,77,79,81,89,92,99.
There are 12 number of data.
Dividing the set into quartiles, each quarter will have 3 terms as

We need to find lower Quartile and upper Quartile to compute the interquartile range.


The formula interquartile range is




Hence the interquartile range of the given set is 19.5.