same enzyme data used in Figure 9-7 is shown in Figure 9-8a.

© John Wiley & Sons, Inc.

FIGURE 9-8: Box-and-whiskers charts: no-frills (a) and with variable width and notches (b).

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.

Software can provide various enhancements to the basic box plot. Figure 9-8b illustrates two

such embellishments you may consider using:

Variable width: The widths of the bars can be scaled to indicate the relative size of each group.

Notches: The box can have notches that indicate the uncertainty in the estimation of the median. If

two groups have non-overlapping notches, they probably have significantly different medians.