Clinical Trials

❘❙❚■ Chapter 36 | Use of FiguresFigure 7. A truncated box and whisker plot to show the CD4 count at the end of 48 weeks’ follow-up forpatients in the MaxCmin 1study.1,2001,000CD4 count at week 40 (cells/mm 3 )8006004002000r = ritonavir.Indinavir/r(n = 143)Saquinavir/r(n = 133)From Figure 7, you can see that there are no substantial differences in the CD4count levels between the two treatment arms at week 48. The median measurementsof both box plots are displaced to the bottom of the box, revealing a positive skewof the CD4 count data. This is emphasized by the shorter lower whisker and by thepresence of outliers above the limits of the upper whiskers. Box plots can also bepresented horizontally rather than vertically. They are simple to produce anduseful for identifying outliers.HistogramsHistograms look like bar charts (Figure 8). They are constructed in a similarfashion, but are used for continuous data rather than discrete numeric orcategorical data. In order to create a histogram, data need to be separated intocategories or bins where all the categories normally have equal width. The widthof each bar relates to a range of values for that variable, and the area of the baris proportional to the frequency of observations in that range. After definingthe categories, the construction of a histogram is essentially the same as that ofa bar chart.416