Statistical graphs – Bar Graph or Chart

Introduction

Having organized the data into a frequency distribution, you need to present them in graphical form. The purpose of graph in statistics is to convey the data to the viewers into pictorial 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.

Bar Graphs:

When the data are qualitative or categorical, bar graphs can be used to represent the data. It is a diagram consisting of a series of horizontal or vertical bars of equal width. We note that the length or height of each bar is equal to the frequency it represents and the bars are equally spaced.

Definition: A bar graph represents the data by using vertical or horizontal bars whose heights or lengths represent the frequencies of the data.

NOTE:

Bar graph may be represented VERTICALLY OR HORIZONTALLY.

Ø  Vertically Representation; the class interval is plotted on the horizontal axis and the frequency on the vertical axis as the diagram below:

Ø  Horizontal Representation; the class interval is plotted on the vertical axis and the frequency on the horizontal axis as the diagram below:

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

Example one:

The following table shows the interval of a rainfall pattern in a certain town.

Year	1991	1992	1993	1994	1995	1996
Rainfall	30.5	43.0	66.0	58.0	49.5	31.0

1.      Draw a bar graph for the data.

2.      Determine the modal rainfall year.

SOLUTION

Steps:

A.    Draw and label x and y axes. For the horizontal bar graph place the frequency scale on the x axis, and for the vertical bar graph place the frequency scale on the y axis.

B.     Draw the bars corresponding to the frequencies as below.

1.      The modal rainfall year is 1993.

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