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Parallel Boxplots

With parallel boxplots (aka, side-by-side boxplots), data from two distributions are displayed on the same chart, using the same measurement scale.

The parallel boxplot below summarizes results from a medical study. The treatment group received an experimental drug to relieve cold symptoms, and the control group received a placebo. The boxplot shows the number of days each group continued to report symptoms.

Control group
Treatment group
2 4 6 8 10 12 14 16

Neither distribution has unusual features, such as gaps or outliers. Both distributions are skewed to the right, although the skew is more prominent in the treatment group. Patient response was slightly less variable in the treatment group than in the control group. In the treatment group, cold symptoms lasted 1 to 15 days versus 3 to 17 days for the control group. The median recovery time is more telling - about 5 days for the treatment group versus about 9 days for the control group. It appears that the drug had a positive effect on patient recovery.

See also:   AP Statistics Tutorial: Comparing Distributions | AP Statistics Tutorial: Boxplots