Reading a Bar
GraphScale of the Frequency Axis

Since bar graphs visually display information, the starting point and interval of the scale are very important. Choosing the scale is much more important than simply making the graph fit on the paper. The starting point and interval can be changed to make a graph look different, although the data it is presenting is actually unchanged. For example, scale can be altered to make bar heights seem dramatic or to make them seem insignificant.

 Where Does the Scale Begin?

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We should take a more in-depth look at scale to demonstrate some of the things you should be looking for when evaluating a bar graph. The scale is determined by the range of values being represented. This means that the scale does not need to begin at zero.

Both graphs below contain the same information. Compare the two. Look at how the scale of the frequency axis can make a difference in the presentation of the information.

 Sales for Company X for the
Fourth Quarter-1995

 Sales for Company X for the
Fourth Quarter-1995

 Note the scale in the graph above:

  • The frequency scale begins at 74 and ends at 82. This is because the range of values being graphed are from 77 to 82. Since we want our smallest bar to have some height, we started the scale at 74.
  • If you look at bar heights, it looks like the ratio is 2:1. However, if you use the actual values, the ratio is then 82:77, which is not 2:1.
  • Direct comparison of bar heights can only be made when the scale begins at zero. So why not begin all scales at zero?

 Note the scale in the graph above:

  • The frequency scale with begins at zero.
  • It is clear that the bars for Oct and Dec have nearly the same height. This means ratio of Oct. sales to Dec. sales is nearly 1:1
  • Having all bars appear nearly the same height makes it difficult to analyze any trends that may exist or to look at differences between months.

 Interval of the Scale

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The interval of scale is the amount from one tick mark to the next along the axis. Just as changing where the scale begins and ends (range of values), changing the interval of the scale also affects the visual look of a bar graph. Below is an example of how two bar graphs that present the same information can appear different.

Suppose a store manager wants to entice her employees to make more sales. Most sales people usually make around 40 sales per month but the manager wants each person to increase sales to 50 per month. She decides to present a bar graph that shows the average commission made per month on sales. Which graph below will more likely provide more incentive for increasing sales?

Averages Sales Commission Earned
on Number of Sales per Month

Notice that Figure B makes the increase in commission from $100 to $170 look bigger than in Figure A. The store manager would want her employees to see Figure B. It would be more effective in enticing employees to strive for 50 sales per month.

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