Statistical graphs – Pie Graph

Introduction

The purpose of graph in statistics is to convey the data to the viewers into pictorial form. That is after you have organized the data into a frequency distribution, you need to present them in graphical form. The reason is that it is easier for most people to understand the meaning of data presented graphically than data presented numerically in frequency distributions or tables. This is especially true if users have little or no statistical knowledge.

Uses of statistical graphs

1.      Statistical graphs can be used to analyse a data or described it.

2.       They are used to get the audience’s attention in a speaking presentation.

3.      They are also used to discover a pattern in a situation over a period of time.

4.      Statistical graphs are used to summarize a data.

The Pie chart or Graph:

This is a graph divided into sectors, each sector representing a different value of category. We note that, the angle of each sector is proportional to the value of the part of the data it represents. Pie graphs are used extensively in statistics. The purpose of the pie graph is to show the relationship of the whole by visually comparing the sizes of the sections. Percentages or proportions can be used. The variable is normal or categorical.

Definition: A pie graph is a circle that is divided into sections or wedges according to the percentage of frequencies in each category of the distribution.

The example below shows the procedure for constructing a pie graph.

Example one:

The table below shows the number of students who offer certain subjects.

SUBJECT	NO. OF STUDENTS
Mathematics	45
Physics	39
Chemistry	28
Biology	14
Economics	36
History	18

a)      Draw a pie graph to illustrate the above information.

b)      What percentages of the number of students offer mathematics?

SOLUTION

STEPS:

Step 1. Since there are 360⁰ in a circle, the frequency for each class must be converted into a proportional part of the circle. The conversion is done by using the formula;

where f = frequency for each class and n = sum of frequencies. Hence the following conversions are obtained. The degrees should sum up to 360⁰.

Step 2. Each frequency must be converted to a percentage. This is done by using the formula:

 Hence, the following percentages are obtained. The percentages should sum up to 100%.

Step 3. Next, using a protractor and a compass, draw the graph using the appropriate degree measures found in step 1, and label each section with the name and percentages, as shown below:

a.       From the table in step 2, the percentage of students who offered mathematics is 25%.

For any further advice on your studies or any challenge on this post, please feel free to contact me at: dzaazo@gmail.com. I will appreciate it.