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Quartiles are the percentage points that break down the ordered data set into quarters.

Garth Frazer, PPG1004 2008 


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.  


Interquartile range

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.

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