QUARTILES

Quartiles can be used to measure spread and include all of the data. The data is simply divided into quarters:

QUARTILES: Summerton versus Williamville
Summerton sales
Williamville sales
250
100
300
200
350
250
350
300
400 QUARTILE 1 - THE LOWER QUARTILE 300
400
350
400
400
450
450
450
450
450 QUARTILE 2 - THE MEDIAN QUARTILE 450
500
500
500
550
600
550
650
600
700 QUARTILE 3 - THE UPPER QUARTILE 600
750
650
1000
700
1200
700
1500
750



Quartiles can be found with the following formulae:
QUARTILE 1: Q1 = 0.25*(n+1) = 5 ie the 5th number QUARTILE 2: Q2 = 0.5*(n+1) = 10 ie the 10th number QUARTILE 3: Q3 = 0.75*(n+1) = 15 ie the 15th number The quartile representation of the data clearly shows where the major differences are between
the 2 sets of data. Quartile 1 and quartile 3 for Summerton is substantially higher than that for
Williamville whereas around the centre line, quartiles 2 are very similar.

The quartile measure does not pick up the large differences at each end of the data (outliers).
Therefore, the 2 suburbs although reasonably comparable in the middle range, are not comparable
at the outer ranges.