CHAPTER 9 Summarizing and Graphing Your Data 127

Box-and-whiskers charts

The box-and-whiskers plot (or B&W, or just box plot) plot uses very little space to

display a lot of information about the distribution of numbers in one or more

groups of participants. A box plot of the same enzyme data used in Figure 9-7 is

shown in Figure 9-8a.

Looking at Figure 9-8a, you notice the box plot for each group has the following

parts:»

» ^{A box spanning the interquartile range (IQR), extending from the first quartile}

of the variable to the third quartile, thus encompassing the middle 50 percent

of the data.»

» ^{A thick horizontal line, drawn at the median, which is also the 50th centile. If}

this falls in the middle of the box, your data are not skewed, but if it falls on

either side, be on the lookout for skewness.»

» ^{Lines called whiskers extending out to the farthest data point that’s not more}

than 1.5 times the IQR away from the box, and terminate with a horizontal bar

on each side.»

» ^{Individual points lying outside the whiskers, which are considered outliers.}

Box plots provide a useful visual summary of the distribution of each subgroup for

comparison, as shown in Figure 9-8a. As mentioned earlier, a median that’s not

located near the middle of the box indicates a skewed distribution.

Some software draws the different parts of a box plot according to different rules,

so you should always check your software’s documentation before you present a

box plot so you can describe your box plot accurately.

FIGURE 9-8:

Box-and-whiskers

charts: no-frills (a)

and with variable

width and

notches (b).

© John Wiley & Sons, Inc.