2.1 Quartiles from a Simple Data Set

As with the mean, there is a measure of dispersion associated with the median, but before we can find this we have to know about quartiles.

Quartiles are those members of the data set which split the data (when taken in order of size) into quarters. Thus we have:

The procedure is to find and isolate the median, and then to find the median of each half of the remaining data.
This ensures that the same number of data values lie within each ‘quarter’ of the data set.

A few examples help. Each X represents a member of the data set. the positions of the quartiles are marked.
The median is, of course, the same as the middle, or second, quartile.