Quartiles are descriptive statistics and can be attributed as first, second and third. For a given dataset, the first quartile is the value greater than 25% but smaller than 75% of the data. The second quartile is the value greater than and smaller than 50% of the data (i.e., the median). The third quartile is the value greater than 75% and smaller than 25% of the data. Equivalently, the first quartile is the 25th percentile, the second quartile is the 50th percentile, and the third quartile is the 75th percentile.

Example 1:

For the dataset of 6, 2, 13, 17 and 22. The 1st and 3rd quartiles are 5.75 and 17.25, respectively. Note the percentage of data fall below 5.75 = 1/5 = 25% and the percentage of data fall below 17.25 = 4/5 = 75%.

Example 2:

For a dataset of 6, 2, 13, 17, 22 and 900. The 1st and 3rd quartiles can be calculated as 6 and 22, respectively.

Then, the percentage of data fall below 6 = 1.5/6 = 25%. Note the number "6" is split half. Similarly, the percentage of data fall below 22 = 5.5/6 = 75%.