Quartiles divide a rank-ordered data set into four equal parts.

The interquartile range (IQR) is a measure of variability, based on dividing a data set into quartiles.

Due to its resistance to outliers, the interquartile range is useful in identifying when a value is an outlier.

**Formula to calculate interquartile range.**

Q3 is the median of the upper quartile.

Q1 is the median of the lower quartile.

**Example:**

Suppose the median of the upper quartile is 50 and the median of the lower quartile is 25, calculate the interquartile range.

Therefore, the IQR is 25.