What Is Quartile?
Quartiles divide a sorted data set into four equal-sized groups. Q1 (the first quartile) marks the boundary below which 25% of the data falls, Q2 is the median (50%), and Q3 marks the 75% boundary. The gap between Q1 and Q3, called the interquartile range, is a common way to measure spread while ignoring extreme outliers.
Example
For the sorted data set 1, 3, 5, 7, 9, 11, 13: Q2 (the median) is 7, Q1 is 3, and Q3 is 11.