The result of dividing a range into four sections with an equal number of items in each section when sorted. For example, if your (sorted) data values are 1.3, 1.7, 2.2, 2.5, 2.6, 2.6, 2.7, 2.9, 3.0, 3.1, 3.3, 3.4, then you divide these into four equal sections: 1.3, 1.7, 2.2 | 2.5, 2.6, 2.6 | 2.7, 2.9, 3.0 | 3.1, 3.3, 3.4; and the quartiles are therefore 2.35, 2.65, and 3.05. The middle of these is called the median and the other two are the lower and upper quartile respectively. Of the data, 25% of values are smaller than the lower quartile, 50% smaller than the median and 75% smaller than the upper quartile.
Used on pages 52, 53