APPENDIX VI: Putting Error Bars on Graphs
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When displaying mean values on scatterplots or bar graphs, it's almost never OK to display the mean values without somehow representing spread or variation in the data. Just as you (or a statistical test) need to know something about variation in the data to determine whether a mean value of 5 is meaningfully different from a mean value of 7, a graph should display variation around mean values so that viewers can correctly interpret possible differences between those means.

The examples below give some tips for using error bars (as opposed to graphing all raw data) to represent variation around mean values. The first example is for a scatterplot, the second for a bar graph. These examples assume that you already know how to make these basic types of graphs. If you need help getting started with your graphs, try these links for basic directions: scatterplot, bar graph, graph formatting.

Scatterplots vs. Bar Graphs
By convention, scatterplots are used for displaying data that are numerical measurements such as size, weight, etc. By contrast, bar graphs are typically used for displaying data that represent counts, percentages, or frequencies.

What Values to Use for Error Bars An Example Using a Bar Graph

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