Pie Charts and Proportionally Divided Circles

Pie Charts

Pie Charts are very useful for showing size proportions of different categories in a data set. They are visual diagrams, that are common, easy to interpret and aesthetically pleasing when coloured; pie charts are often accompanied with a key to show what the different sections represent.

The data must first be converted into percentages before it can be plotted onto a pie chart diagram. These percentages are then coverted into a proportion of 360 (ie the total number of degrees in a circle), this is done by multiplying the percentage values by 3.6. Remember we need to plot the percentage values that we have as a proportion of the circle, say we have a value of 13.5% we need to find out how may degrees 13.5% of 360 is. We divide 360 by 100 to find what 1% is (ie 3.6) and then multiply it by the percentage we have. This then gives us the number of degree of the circle that the category takes up, which can be plotted using a protractor.

Pie charts are useful for making comparisons in the values of different categories, when using one diagram, as well as comparing different data sets, by using more than one diagram; however it is hard to compare a lot of diagrams that are divided several times. Pie charts generally don't tend to show exact figures, unless they are written on the diagram after the pie chart is constructed, meaning it can be hard to make detailed comparisons and quote actual figures.

Proportionally Divided Circles

Thses are the same as pie charts, but the area of the circle is proportional to the total value of the data set. These are harder to construct and pointless when only using one diagram, but they do allow more detailed comparisons to be made between data sets as they provide an indication of differences in the total value of the data sets.

Both type of diagram must be large enough for the proportions of different categories to be clear and visible, small diagrams are very hard to interpret.