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Sector (pie) graphs

Sector graphs are also called pie charts or pie graphs. Sector graphs are like divided bar graphs. Both types of graphs show a total amount split into proportional parts. Sector graphs use a circle. Each quantity is graphed as a sector of the circle. The size of the sector depends on the fraction it is of the whole.

Pie charts are best used for displaying statistical information when there are no more than six components only—otherwise, the resulting picture will be too complex to understand. Pie charts are also not useful when the values of each component are similar because it is difficult to see the differences between different segments.
 
When a pie graph is created, each sector is constructed as a fraction of the whole circle (a circle is equal to an angle of 360o).

Example:

A group of 100 students who belonged to a school pet club were asked to name their favourite animals. The results of the survey were then displayed in a pie graph.

The graph shows that:

50% (50 students) of the 100 club members had a preference for dogs (180o).
25 of the students preferred cats (90o).
10 percent, or 10 students, each liked birds or guinea pigs (36o).
The other 5 students chose mice as their favourite pet (18o).

 
Drawing a pie-chart
 
To draw a pie graph, you will need to have the following stationary ready:
 
·         Compass (for drawing circles accurately)
·         Protractor (to measure angles)
·         Ruler
·         Calculator (if deemed necessary)
 
For example to draw a pie chart representing the popularity of various colours among people, you need to take the following steps:
 
1.       Study the data given:
 
 
Colours
No. of People
Red
4
Orange
3
Yellow
2
Green
1
 
 
2.       Convert the given data into percentage (%) form:
 
Colours
No. of People
%
Red
4
40%
Orange
3
30%
Yellow
2
20%
Green
1
10%
 
 
3.   In order to draw a pie graph, the percentage figures will need to be changed into angles. This is because a pie-chart is essentially a circle divided into segments. So in order to be able to draw the different segments, you would need the angle. To achieve this, you would apply the following formula:
 
= x% x 360° (a full circle = 360°)
= x/100 x 360°
= angle
 
 
Colours
No. of People
%
Angle
Red
4
40%
144°
Orange
3
30%
108°
Yellow
2
20%
72°
Green
1
10%
36°
 
4.   To be sure your calculations are correct, you can simply add up the angles. The total should add up to 360°.
 
 
5.   To start drawing the pie graph, begin by drawing a circle on a piece of paper. This is best done using a compass as it will ensure accuracy and evenness.
 
 
6.   From the centre of the circle, draw a horizontal line to the edge of the circle (this is called the RADIUS). This will serve as a base line to construct the pie graph.
 
 
7.   Measure the largest angle in the data with the protractor, starting at the baseline, and mark it on the edge of the circle. To do this, place your protractor on the circle so that the 90 degrees are directly above the center of the circle. Use the ruler to draw another radius to that point. Use this new radius as a base line for your next largest angle and continue this process until you get to the last data point. You will only need to measure the last angle to verify its value since both lines will already be drawn.
 
 
8.   When drawing a pie chart, ensure that the segments are ordered by size (largest to smallest) and in a clockwise direction.
 
 
9.   Label and shade the sections of the pie chart to highlight whatever data you may wish to showcase. You may also include a legend if that is useful.

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