Clinical Trials

❘❙❚■ Chapter 17 | Types of Data and Normal DistributionFigure 3. Box plots of systolic blood pressure (SBP) data from Table 2.250200SBP (mm Hg)15010050A P0Visit 1A PVisit 2A PVisit 3A PVisit 4A = active treatment; P = placebo.The first is the number of observations together with the number of missingobservations. This sample size information is important in determining thestrength of observed evidence and illustrating the dynamics of patient follow-up.The second group of summary measures includes the mean and standarddeviation. The former is the average level of SBP for the respective group, whichshows that, after the baseline, patients on the study drug have a consistently lowerSBP than the placebo group. The standard deviation provides a measure of thespread of SBP for the respective groups.The third category of summary measures are three percentiles. The 25thpercentile of SBP for the active treatment group at visit 2 is 110 mm Hg: 25% ofpatients had an SBP below 110 mm Hg (in the placebo group, 25% of patients hadan SBP below 116 mm Hg). Table 2 shows that the active treatment group hasa consistently lower SBP than the placebo group after randomization in termsof the 25th, 50th, and 75th percentiles.The SBP data in Table 2 can also be summarized visually in a box plot(Figure 3), in which the vertical axis represents the SBP and the horizontal axisrepresents the factors of interest (treatment and visit). The box plot shows themain body (covering 50% of data values around median with the top of the boxmarking the 75th percentile and the bottom the 25th percentile) as well asindividual values outlying the main body. This box plot, comparing SBP bytreatment and visit, shows that the study drug has some effect on SBP afterrandomization with respect to both location and variation.174