interquartile range

In statistics, a measure of the range of data, equalling the difference in value between the upper and lower quartiles. It provides information on the spread of data in the middle 50% of a distribution and is not affected by freak extreme values.

For example, in a survey carried out to determine how much time pupils spent on homework each week, the interquartile range was six hours. This meant that the middle 50% of the pupils varied by six hours in the amount of time they spent on homework. If the interquartile range had been just one hour this would have indicated that most pupils were spending the same amount of time on their homework. In other words the interquartile range provides information on how much variation there is in the amount of time pupils spend on homework, but it does not look at the extremes, such as those that spend virtually no time on their homework or those that spend a large amount of time doing homework.

Often the interquartile range is found using a cumulative frequency diagram.