Also known as a parallel boxplot or comparative boxplot, a side-by-side boxplot is a visual display comparing the levels (the possible values) of one categorical variable by means of a quantitative variable. As its name implies, the side-by-side boxplot is constructed by placing single boxplots adjacent to one another on a single scale.
A side-by-side boxplot has all the advantages of a single boxplot (which can be seen here) with the added benefit of providing clear comparisons between levels in:
- Maxima and Minima
The simplicity and comparative power of the side-by-side boxplot is the reason it is used much more often than the single boxplot. (In fact, a google image search of the word “boxplot” actually shows more side-by-side boxplots than single boxplots.)
Side-by-side boxplots are fairly straightforward and contain a large amount of information considering how simple they are. However, some data are better represented by a side-by-side boxplot than others; skewed data are particularly bad (as seen in Figure 2). Additionally, the formatting of the graph can be misused to make good side-by-side boxplots misleading; here is a site that illustrates this.