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Pie Chart

Pie charts are a typical type of data representation where data is represented as a part of a circle. The circle represents the total value (or 100%), and the different parts represent certain proportions (or percentage) of the total. In a pie chart, the arc length of each sector (and in turn its central angle and area), is proportional to the part it represents.
The origin of the pie chart is traced back to Florence Nightingale in 1858. This was the year when she presented a paper on the causes of deaths in her army in the eastern part of the world.
Following is the pie chart originally developed by Florence Nightingale in the year 1858. (Actually called by her as the ‘Polar Area Diagram’.)

The below graphic gives the number of deaths that occured from diseases that could have been prevented (in light grey), those that were the results of wounds (in dark grey) and those due to other causes (in black).


There are two approaches constructing a pie chart from any given data:
  1. Degree Approach:
    The central angle in a circle represents 360o, so any part or segment in a pie chart is calculated as a proportion of 360o.
  2. Percentage Approach:
    In this case, any part or segment in a pie chart is calculated as a part of 100%.
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If we convert the same pie chart into the degree format, we will be required to do the following conversions:
Total = 100% = 360o
Hence 1% = 3.6o
Central = 10% = 36o
North = 20% = 72o
South = 25% = 90o
East = 15% = 54o
West = 30% = 108o

Limitations of Pie Charts

Despite the pie chart being one of the most important ways to represent data, it is marred by limitations of its own:
  • Pie charts can be used only when the sum of all categories is given, for example if the categories represent proportions or percentage of a total.
  • A single pie chart can represent only one continuous variable.

Significance of Pie Charts

The pie chart has gained prominence due to the following reasons:
  • In a pie chart, we get a clear picture of the contribution of different sectors to the build up of the total. E.g., presentation of budgets.
  • Comparing two pie charts is easier than comparing two bar charts or any other format of data representation.
Let us see the following data:
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Description: 44372.png 
Sales value in 2005 = Rs 180 crores
Sales value in 2006 = Rs 204 crores
What is the percentage increase in the sales value of the East zone?
There are two percentage increases (A) The total sales value of company XYZ is increasing. (B) The percentage contribution of the East zone is increasing.
Percentage increase in The total sales value of the company XYZ = 30%
Percentage increase in the percentage contribution of the East zone = 20%
Hence, the net percentage increase = 56%
(Successive increase of 20% and 30%)

Types of Pie Charts

There are two types of pie charts:
  1. Normal Pie Chart
    This displays the contribution of each component of the pie.
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  1. Exploded Pie Chart
    This pie chart has all the characteristics of a normal pie chart, the only addition is that the contribution of individual segments is highlighted.
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