Just as one median divides the data into two subgroups, three quartiles divides the data into 4 quarters. The three quartiles are normally represented by the points Q1, Q2, Q3
The lower quartile is denoted by Q1 and the upper quartile by Q3. The second quartile Q2 is the same as the median.
Positions of the Quartile
Lower Quartile Q1 (25 th percentile) : (n+1)/4
Upper Quartile Q3 : 3(n+1)/4
The difference between the upper and the lower quartile value is called the Interquartile range