Clinical Trials

Clinical Trials: A Practical Guide ■❚❙❘Figure 2. Inverse normal plots for the four outcome variables given in Table 1.Pretreatment FEV 1 for APretreatment FEV 1 for B2,5002,500FEV 12,000FEV 12,0001,5001,5001,500 2,000 2,500Inverse normal1,500 2,000 2,500Inverse normalPosttreatment FEV 1 for APosttreatment FEV 1 for B3,0003,0002,5002,500FEV 12,000FEV 12,0001,5001,5001,500 2,000 2,500 3,000Inverse normal1,500 2,000 2,500 3,000Inverse normalmeaning that there is no difference in pre- and posttreatment population meansof FEV 1in group A. The alternative hypothesis is:H a: μ d= μ pre– μ post≠ 0.The test statistic for testing the above hypotheses is given by:X dt =SE(Xd )(4)SE(X d) = S d/ √nwhere:• X dis the sample mean of the paired FEV 1difference• S dis the standard deviation of the paired FEV 1difference• n is the number of pairs of observations (in this case, the number of patients)• SE(X d) is the standard error of X d205