Ordering
Sometimes the categories in a table or bar chart have a natural ordering. For example,
In such tables, the categories should be kept in this natural order.
When there is no natural ordering of the categories, the order of the categories in a table or bar chart is arbitrary. For example, in a frequency table (or the corresponding bar chart) of causes of death, there is no natural way to order the causes (heart disease, stroke, accident, ...) so they can be placed in any order on a bar chart.
Alphabetical ordering of the categories is rarely best.
Ordering by size
When there is no natural ordering of categories, it is often useful to arrange them in decreasing order of their values. The larger values are often more 'important' in some way, so this ordering helps them to stand out.
When this type of ordered bar chart is drawn for values that are a partition of some total, it is often called a Pareto diagram. It is named after an Italian economist in the late 1800's who found that about 80 percent of the wealth of a region was concentrated in less than 20 percent of the population. (Pareto diagrams are particularly useful in industrial quality control and quality improvement where information is collected about the causes of problems in manufacturing processes.)
However if there is a category 'Other' or 'Unknown', then it should be kept at the end, even if its value is larger than others.
R&D expenditure in OECD
The barchart below shows the expenditure on Research and Development in all OECD countries in 2005 (or the latest available figures), expressed as a percentage of Gross Domestic Product (GDP).
The countries are initially arranged alphabetically. Click the checkbox Sorted values to rearrange them in decreasing order of R&D expenditure. In the reordered table, it is easier to identify the countries with highest and lowest R&D expenditure and to understand the rank of individual countries.
Cumulative sums
A Pareto diagram is often augmented by a line showing the proportion of all categories up to and including the current one.
Advertising expenditure in New Zealand
The diagram below shows the amounts spent on advertising in different media in New Zealand in 2004. The media have been ordered by expenditure, so the most 'important' media are on the left. The line shows the cumulative proportion of expenditure for each type of media and those more important.
Click on the bar for Radio to stack the bars for Newspapers, Television and Radio. The cumulative proportion line goes through the top of this stack, so it shows the proportion of advertising expenditure for these three types of media. Click on other bars to read off other cumulative proportions.
Finally, click the checkbox Separate scale for cumulative propns to expand the scaling of the individual bars of the bar chart and therefore make comparisons easier. Note that a different scale is used for the cumulative proportions (on the right) and the individual proportions (on the left).