Learning Statistics with R - A tutorial for psychology students and other beginners, 2018a

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(a)
(b)
Figure 6.11: A basic boxplot (panel a), plus the same plot with annotations added to explain what aspect
of the data set each part of the boxplot corresponds to (panel b).
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> stem( x = afl.margins, scale = .25 )
> stem( x = afl.margins, width = 20 )
The only other thing to note about stem and leaf plots is the line in which R tells you where the decimal
point is. If our data set had included only the numbers .11, .15, .23, .35 and .59 and we’d drawn a stem
andleafplotofthesedata,thenR would move the decimal point: the stem values would be 1,2,3,4 and
5, but R would tell you that the decimal point has moved to the left of the | symbol. If you want to see
this in action, try the following command:
> stem( x = afl.margins / 1000 )
The stem and leaf plot itself will look identical to the original one we drew, except for the fact that R
will tell you that the decimal point has moved.
6.5
Boxplots
Another alternative to histograms is a boxplot, sometimes called a “box and whiskers” plot. Like
histograms, they’re most suited to interval or ratio scale data. The idea behind a boxplot is to provide
a simple visual depiction of the median, the interquartile range, and the range of the data. And because
they do so in a fairly compact way, boxplots have become a very popular statistical graphic, especially
during the exploratory stage of data analysis when you’re trying to understand the data yourself. Let’s
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