Proportions within groups

Although a contingency table fully describes categorical data from two or more groups, the raw frequencies are a poor way to compare the distributions if there are different total numbers in the groups.

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Blood type and race

Since it is easier to compare values down the columns of a table than across rows, a table of proportions is usually best presented with the groups corresponding to rows of the table, rather than columns.

Bar charts of proportions

Bar charts provide a graphical way to compare groups. Although the bar chart of each group has the same shape whether it is based on frequencies or proportions, comparisons are made more easily if proportions are used, especially when the groups are of different sizes.

Dolphin activity

Clustering the bars

When separate bar charts are drawn for the different groups, the bars for any category are widely separated in the diagram, making comparisons harder. An alternative display uses the same bars, but clusters them by the values of the categorical variable, rather than by groups. This type of clustered bar chart makes it easier to spot subtle differences between the groups.

Blood type and race