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Pie charts are used to display data as a percentage of a whole. Any time a pie chart is drawn, the percentages must be calculated from the data set that is being used. There is a quick method for figuring out percentages in pie charts.

Start by getting the sum total of all the data. For example, we'll use the following data from the construction industry obtained from the Department of Labor Statistics: carpenters, 805,410; construction labourers, 836,420; construction managers, 179,900; electricians, 477,540; and other equipment operators, 273,050. For this data, the total is 2,572,320.

Divide the value for each category by the final total to get the percentage for that category in decimal form. Multiply the decimal form by 100 to get the percentage form.

Carpenters: 805,410/2,572,320 = 0.313 x 100 = 31.3 per cent Construction labourers: 836,420/2,572,320 = 0.325 x 100 = 32.5 per cent Construction managers: 179,900/2,572,320 = 0.070 x 100 = 7.0 per cent Electricians: 477,540/2,572,320 = 0.186 x 100 = 18.6 per cent Other equipment operators: 273,050/2,572,320 = 0.106 x 100 = 10.6 per cent

Make sure the percentages total 100. If they don't, rework the calculations to find the error. (31.3 + 32.5 + 7.0 + 18.6 +10.6 = 100)

Use the percentages to calculate the angles necessary to draw the pie chart.

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- Rather than calculating pie chart percentages manually, enter the data into a spreadsheet program. Use the "Chart" command to create the chart you need.

- pie graph image by Tomislav from Fotolia.com