Ordering categories

If the categories have a natural ordering (an ordinal categorical variable), this ordering should be used in a bar chart.

For nominal categorical variables (no natural ordering), alphabetic ordering of the categories should be avoided. It is better to sort them in order of decreasing frequencies, giving a Pareto diagram.

Detecting 'important' categories

Pareto diagrams are particularly useful in industrial quality control and quality improvement where information is collected about the causes of problems in manufacturing processes. The Pareto principle states that:

A large percentage of instances of any problem result from a small percentage of the possible causes.

The leftmost categories in a Pareto diagram are most important. A line is usually added showing the cumulative proportions for the different causes. For the i'th category, the height of the line gives the proportion of problems from any of the i most common categories.