Pie Chart

A pie chart is a graphical representation of data as a disk divided into wedge-shaped ‘pieces’ or ‘slices’ whose sizes are proportional to the relative frequencies of the categories they represent. Because pie charts are most useful when they include only a small number of categories, they are most often used to display the relative frequencies of a categorical data set. Pie charts are a logical choice for graphical presentation when the focus of interest is on the relative frequencies of the data—that is, how much of the whole each category represents, rather than the absolute frequency of each category (in the latter case, a bar chart would be more appropriate).

Florence Nightingale, a founder of modern nursing, was also well versed in statistics. She invented a type of pie chart to show that in the Crimean War, far more soldiers died of illness and infection than those who died of battle wounds. Her campaign succeeded in improving hospital conditions and nursing so that many lives were saved.

Pie charts are most often created using statistical software but may also be created by hand: to obtain the angle for any category—that is, the size of the ‘slice,’ we multiply the relative frequency by 3608, because there are 3608 in a complete circle.

For example, there were 863 kidney transplant patients who had their transplant performed at the Ohio State University Transplant Center during the period 1982 to 1992. There were 432 white males, 92 black males, 280 white females, and 59 black females.

Table 1 gives the race and sex information and calculates the relative frequencies for the four response categories.

To make a pie chart for this data set, we need to divide a disk into four wedge-shaped pieces that comprise 50.1%,10.7%,32.4%, and 6.8% of the disk. We do so by using a protractor and the fact that there are 3608 in a circle. Thus, four pieces of the disk are obtained by marking off 180.368, 38.528, 116.648, and 24.488, which are 50.1% of 3608,10.7% of 3608, 32.4% of 3608, and 6.8% of 3608, respectively. The pie chart for the relative frequency distribution for the above table is shown in Figure 1.

Figure 1 Race and Sex of Kidney Transplant Patients: Demographics of Kidney Transplant Recipients at the Ohio State University Transplant Center (19821992)