Elementary Statstics

5.1.8 Pie diagram

Pie diagrams are very popularly used in practice to show percentage breakdowns. For example, with the help of a pie diagram we can show how the expenditure of the Government is distributed over different heads like Agriculture, Irrigation, Industry, Transport, Defence, etc. Similarly through a pie diagram we can show how the expenditures incurred by an industry are divided under different heads like raw materials, wages and salaries selling and distribution expenses, etc. The pie chart is so called because the entire graph looks like a pie, and the components resemble slices cut from pie.

While making comparisons, pie diagrams should be used on a percentage basis and not on an absolute basis, since a series of pie diagrams showing absolute figures would require that larger totals by represented by larger circle. Such presentation involves difficulties of two-dimensional comparisons. However, when pie diagrams are constructed on a percentage basis, percentages can be presented by circles equal in size. It may be noted that this problem does not arise in the use of a single pie diagram.

In laying out the sectors for pie chart, it is desirable to follow some logical arrangement or sequence. It is common practice to begin the largest component sector of a pie diagram at 12 O’clock position on the circle. Usually the other component sectors are placed in clockwise succession in descending order of magnitude, except for catch-all components like ‘miscellaneous’ and ‘all others’ which are shown last, contrast with adjacent sectors.

In constructing a pie chart the first step is to prepare the data so that the various component values can be transposed into corresponding degrees on the circle. Suppose there are four components in a series representing the following values : (i) 60 percent, (ii) 25 percent, (iii) 15 percent(360/100=3.6),(iv) 5 percent the corresponding values of the four components in the illustration are (60) x (3.6) = 216; (25) x (3.6) = 90; (10) x (3.6) = 36; (5) x (3.6) = 18.

The second step is to draw a circle of appropriate size with a compass. The size of the radius depends upon the available space and other factors of presentation.

The third step is to measure points on the circle representing the size of each sector with the help of a protractor. The ordinary protractor is based upon a scale in which the total circle is 360 degrees, but it is possible to purchase a protractor in which the entire circle is divided not into 360 but 100 equal parts so that the angle representing any desired percentage can be read directly.

In laying out the sectors for a pie chart it is desirable to follow some logical arrangement, pattern or sequence. For example, it is a common procedure to arrange the sectors according to size, with the largest at the top others in sequence running clockwise. An essential feature of the pie chart is the careful identification of each sector with some kind of explanatory or descriptive label. If there is sufficient room, the labels can be placed inside the sectors; otherwise the labels should be placed in contiguous positions outside the circle, usually with an arrow pointing towards the appropriate sector.