Quartiles are three values that divide a data set into four equal parts. The quartiles are denoted as $Q_1,$ $Q_2,$ and $Q_3.$ The second quartile $Q_2,$ also known as the median, divides the ordered data set into two halves. The median of the lower half is the first quartile $Q_1,$ while the median of the upper half is the third quartile $Q_3.$ The first quartile is also called lower quartile, and the third quartile is also called upper quartile. To find the quartiles of a data set, the values must first be written in numerical order. The difference between the upper and lower quartiles is the interquartile range.