Constructing Bar Graphs

After completing this unit, if you are given a table of information that is to be displayed as a bar graph, you should be able to:

• Identify the bar graph title.
• Identify labels for the bar graph axes.
• Identify information given by a designated bar.
• Determine the scale of a bar graph.
• Construct a bar graph.

Bar graphs are used to help visually represent data so that comparisons and trends will be easier to see. In this section you will learn how to construct bar graphs. We will begin with data presented in a table. (NOTE: The procedure for constructing tables was discussed in the Constructing Tables unit.) We will first outline the elements of a bar graph and then the steps in constructing bar graphs.

Below are the steps to following when constructing a bar graph:

1. Determine the following elements of the bar graph from the table
   • Title of the graph.
   • Label for each axis--Here we must determine which is to be the frequency axis and which is to be the grouped data axis.
   • Scale for each axis--Determine the numerical scale for the frequency axis, then the group names for grouped data axis.
2. Draw a set of axes that you will use to construct your graph
   • Determine which axis will be the frequency axis--Determine whether bars will go horizontally or vertically.
   • Write in axes labels.
   • For the frequency axis, determine the scale interval.
3. Use the data from the table to draw in the bars on the graph.

Now let's go through the process of constructing a bar graph using the table titled Student Housing at Union University.

1. Determine the following elements of the bar graph
   • Title of the graph --The title of the bar graph is the same as the title of the table. The title of this graph will be Student Housing at Union College.
   • Label for each axis--The label for each axis comes from the column/row labels. In this table the data is placed in columns, so we will use the column labels. Once you know the axes labels, you need to determine which is the frequency axis, and which is the grouped data axis.The axes labels will be Type of Housing, and Number of Students.

   Grouped Data Axis Label--This is the axis that contains the groups. In this case it's the Type of Housing. Frequency Axis Label --This is the axis that contains the frequency counts. In this case it's the Number of Students.

   • Scale of each axis--The scale of each axis is determined by the data for each axis. Let's look at each of these separately.
     
     • Grouped Data Axis Label--Type of Housing
       This data contains four groups. The data is nominal (not ordered) so the groups could be placed in any order on the axis. The four groups on the scale are: (1) Residence Halls, (2) Fraternity/Sorority Houses, (3) Off Campus Apartments, and (4) Off Campus Houses.
       
     • Frequency Axis Label--Number of Students
       As mentioned in the section on scale, the scale of the frequency axis is determined by the range of the data. The data here range from 985 to 3995. We can set the scale to run from 0 to 4000.
       
2. Draw a set of axes that will you will use to construct your graph
   • Determine which axis will be the frequency axis--Will the bars be horizontal or vertical?
     Determining which direction to construct the bars in is a matter of preference. The information contained in the graph will not be changed, but its visual appearance will be altered. In this example we will place the frequency on the vertical axis.
   • Write in axes labels--In the first step we determined the axes labels were:
     • The grouped data axis, which is on the horizontal axis, is Type of Housing.
     • The frequency axis, which is on the vertical axis, is labeled Number of Students.
   • For the frequency axis, determine the scale interval:

     The interval of scale is the amount from one tick mark to the next along the axis. If the range of the scale is small, a general rule is to take the range of the scale and divide it by 10. Make this your interval. For ranges that are larger, the interval is typically 5, 10, 100, 500, 1000, etc. Use numbers that divide evenly into 100, 1000 (or their multiples) in order to provide a graph that is easy to analyze. In this case, if we take 4000 and divide it by 10 we get 400. However, 500 is a number that is more easily analyzed, so we will use 500. This provides a scale that is not too large and easy to use in analyzing the data.

3. Use the data from the table to draw in the bars on the graph
   Once we have the axes constructed, we are ready to draw in the bars. Using the data from the table, draw in the bars to their appropriate height. Keep in mind that this is somewhat of an approximation since the scale does not make drawing exact heights possible.
   Student Housing at Union University
   

While tables are more exact in their presentation of data, they do not allow the quick visual view of the data. Bar graphs provide one way to present data so that we can get an overview at a glance. In the following practice you will construct a bar graph. While you are doing this, think about the advantages and disadvantages of both representations of data. (Comparison of the different types of visual displays will be outlined in the last instructional unit of this book.)

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