Pie charts are circular graphs divided into sections, or slices, that are used for organizing data. These charts compare the data by using fractions or percentages, with each slice proportional to the fraction or percentage it represents. Pie charts are commonly used in media and corporate reports, especially to summarize budget or financial data. The pie chart is so named because of its resemblance to a sliced pie.
The earliest known use of a pie chart was in an 1805 book by the Scottish mathematician and political economist William Playfair. He used a pie chart to represent the areas of the American states. The Louisiana territory, acquired in 1803, represented the largest slice in the chart.
Pie charts are easy to read and understand if appropriately designed. They are an effective way to present data, especially if the intent is to show one section relative to the whole.
Pie charts are the most commonly used graphic in business reports and presentations. They also are frequently used in journalism. In both cases, they are frequently used to summarize financial data. A pie chart summarizing the federal budget, for example, would use each slice to represent a particular government function and its percentage of the budgetary whole.
Pie charts are rarely, if ever, used in scientific literature. Many scientists and statisticians consider the chart a poor method for summarizing data and point out that it is difficult to compare different sections of a given pie chart. The difficulty in comparing two sections of a pie chart is that it is difficult to see subtle differences in area of slices of a circle as compared to other graphics such as bar graphs.
To make a pie chart readable, it is best to display sections clockwise, from largest to smallest. It also is important to use colors so as to make each section distinct.