The interquartile range shows the size of the span between the middle half of the numbers in a set. The interquartile range is calculated by subtracting the number at the lower quartile from the number at the upper quartile, which mark the first and third quarters of the data set by size respectively.
For the simple data set 1, 2, 3, 4, the lower quartile is 1 and the upper quartile is 4. A quartile consists of one-fourth of the data in a set. The interquartile range is from 1 to 4, which means it is equal to 3. A small interquartile range indicates data with low variance.