The first quartile (Q1) is the 25th percentile. It is the point below which lie 1/4 of the data. The second quartile (Q2) is the 50th percentile. It is the point below which lie 1/2 of the data. This is also called the medium. The third quartile (Q3)is the 75th percentile. It is the point below which lie 3/4 of the data.
The interquartile range is another range used as a measure of the spread. The difference between upper and lower quartiles (Q3–Q1), which is called the interquartile range, also indicates the dispersion of a data set. The interquartile range spans 50% of a data set, and eliminates the influence of outliers because, in effect, the highest and lowest quarters are removed.