Clinical Trials

❘❙❚■ Chapter 36 | Use of FiguresBoth histograms and bar charts display the frequency density, except thathistograms typically do not have spaces between the categories because the dataare continuous. The appearance of a histogram is affected by how the categoriesare defined. An increase in the number of categories reduces the possible numberof observations within the category. The number and width of the categories(bins) should be chosen according to clinically meaningful intervals. If this is notpossible then they should be selected on a trial and error basis, with an overall aimof illustrating variations in the data. As a rough guide, 5–20 categories should beselected, depending on the number of observations in the dataset. This can resultin a loss of information if the width of the categories is too wide; with too narrowcategories, however, the graph will consist of the raw numbers (Figure 8). Notethat most major statistical software packages do a reasonably good job ofautomatically selecting appropriate category widths.Histograms are visually strong and useful for evaluating the spread and skew ofthe data, but are associated with some disadvantages. Firstly, it is difficult tocompare two groups using a histogram; secondly, a histogram tells you how manyvalues lie within a certain range, but without revealing the exact measurements.Scatter plotsA scatter plot is a simple plot used to display the relationship between twocontinuous variables. One variable is termed the ‘x’ and the other the ‘y’. Bothvariables are plotted against each other, the x variable along the x-axis and they variable along the y-axis. The relationship between them can be expressed withthe use of a regression line, which is usually calculated by a software package.A scatter graph can be used to investigate whether there was a relationshipbetween CD4 levels at baseline and week 48 in the MaxCmin 1study (Figure 9).As we would expect, the trend line shows a highly significant relationship betweenthe two variables (Spearman rank correlation coefficient = 0.83, P < 0.0001).On the other hand, a flat trend line does not imply that there is no relationshipbetween the two variables under consideration. A weak relationship might exist,or the trend line might be misleading if its gradient and positioning are the resultof a few influential points. For nonlinear relationships, other lines such assmoothing splines or lowess curves can be used, which try to funnel the points.Line graphsA line graph is a plot of numeric data where consecutive values are connected bya line (Figure 10). They can show repeated measures on a single individual toillustrate how a certain parameter, say the pharmacokinetic (PK) concentration ofa drug, changes over time after it has been administered. In this case, the x-axiswould correspond to the time and the y-axis would reflect the PK concentration.418