Time series
When sets of categorical measurements are recorded at successive times, time can be treated as a grouping variable. Stacked barcharts are often informative displays.
Same-day treatment in hospitals
Binary variables
When the variable of interest can only take two possible values, it is called a binary variable. Examples are
We will call the two possible values success and failure. The proportion of successes is the most useful summary of a binary variable's distribution. Comparison of several groups is based on the proportion of successes in the groups, and these can be displayed in a single bar chart.
Heart disease and snoring