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A box and whisker plot is illustrated in Figure 2.7 for the corn size and treatment group from Farndon et al. (2013). The ‘whiskers’ in the diagram indicate the minimum and maximum values of the variable under consideration. The median value is indicated by the central horizontal line whilst the lower and upper quartiles by the corresponding horizontal ends of the box. The shaded box itself represents the interquartile range. The box and whisker plot as used here therefore displays the median and two measures of spread, namely the range and interquartile range. In Figure 2.7, for the scalpel group the median and lower quartile for the baseline corn size coincide and is 3 mm.

Figure 2.7 Box and whisker plot of size of corn at baseline (in mm) by randomised group for 200 patients with corns.

(Source: data from Farndon et al. 2013).