Deviation bar graph

Synonyms:

back-to-back graph, paired bar charts, population plots, population pyramids

Deviation bar graph with seven red bars to the left of the y-axis and 7 blue bars aligned with the red ones but on the right of the y-axis

Deviation bar graphs are simply two bar charts aligned, where one of the charts runs right to left rather than left to right.

The two charts report on the same categories but differ in terms of the respondent group or some other variable. Thus, the shared categories, such as the survey questions, are listed in the space in the middle of the two bar charts. Usually, these are constructed as bar charts, where the bars are horizontal, not column charts.

The purpose of graphing each set of responses separately from one another is so that it is easier to see the shape of each response set. Thus it is important to order one of the response sets from greatest to least so that it is easier for a viewer to see where the other response set is out of shape.

Examples

Response sets from teachers and principals

Back to back deviation bar graph showing disparity in responses between teachers and principals to a set of statements such as "school climate is positive"

Source: Stephanie Evergreen

This example shows the response sets of teachers and principals to a set of common survey questions. Principal responses are ordered greatest to least, so it becomes obvious that the teachers differed.

Advice for choosing this method

Use this method when you are looking at the overall shape of two response sets, to spot where they did not match. It is not good for comparing the responses between two groups on individual items.

Advice for using this method

Boost the ease of interpretation by colour coding the bars where the responses differ greatly between the two groups.

Resources

Making back-to-back graphs in Excel

This blog post from Stephanie Evergreen of Evergreen Data provides guidance on using Excel to make back to back graphs in order to compare data between responses to the same survey from two different groups.

Other ways to compare sets of values

Bar chart

A bar chart plots the number of times a particular value or category occurs in a data set, with the length of the bar representing the number of observations with that score or in that category.
Block histogram

A histogram is a graphical way of presenting a frequency distribution of quantitative data organised into a number equally spaced intervals or bins (e.g. 1-10, 11-20…).
Bubble chart

Commonly used on maps, and x/y-axis plots, or no plot at all, bubble charts communicate the raw count, frequency, or proportion of some variable where the size of the bubble reflects the quantity.
Bullet graph

Bullet graphs encode a single variable as a bar.
Dot plot

Dot plots encode single data points with circles, often on a line.
Small multiples

Small multiples are an array of graphs on the same scale that are grouped together in a row or grid and are often used to simplify a data display.