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In the previous discussion, we considered the percentiles that divide the data into 100 equal parts. Some of the percentiles have special importance, such as the 25th, 50th, and 75th percentiles. These percentiles are also known as the first, second, and third quartiles (denoted by , and ), respectively. Sometimes, they are also known as the lower, middle, and upper quartiles, respectively. The second quartile is the same as the median. Thus, to determine the values of the different quartiles, one has to simply find the 25th, 50th, and 75th percentiles (see Figure 2.7.1).

Figure 2.7.1 Quartiles and percentiles.