Derive your third quartile based on whether your data set includes an even or odd set of values. With an odd number of values, multiply N by 3, add 1 and divide the result by 4.

There are three quartile values—a lower quartile, median, and upper quartile—which divide the data set into four ranges, each containing 25% of the data points: ...

Box plots characterize a sample using the 25th, 50th and 75th percentiles—also known as the lower quartile (Q1), median (m or Q2) and upper quartile (Q3)—and the interquartile range (IQR = Q3 ...

They found that the spread between top and bottom quartile funds in private equity is a whopping 12.9 percentage points compared to 1.5 percentage points for public equities funds.