Clinical Trials

❘❙❚■ Chapter 36 | Use of FiguresFigure 12. A forest plot showing the overall and individual odds ratios for each study.Steroid Control Weight (%) Mantel–Haenszel Odds ratioodds ratio (95% CI)(95% CI)Ransohoff, 1972 9/17 13/18 3.1 0.43 (0.11, 1.76)Alexander, 1972 16/55 22/55 8.0 0.62 (0.28, 1.36)Faupet, 1976 16/67 16/28 8.9 0.24 (0.09, 0.60)Cooper, 1979 26/49 13/27 4.1 1.22 (0.48, 3.12)Hernesnieml, 1979 35/81 36/83 10.4 0.99 (0.54, 1.84)Pilts, 1980 114/201 38/74 12.4 1.24 (0.73, 2.12)Saul, 1981 8/50 9/50 3.9 0.87 (0.31, 2.47)Braakman, 1983 44/81 47/80 11.1 0.83 (0.45, 1.56)Giannotta, 1984 34/72 7/16 3.1 1.15 (0.39, 3.42)Dearden, 1986 33/68 21/62 5.8 1.84 (0.91, 3.74)Zagara, 1987 4/12 4/12 1.4 1.00 (0.18, 5.46)Gaab, 1994 19/133 21/136 9.2 0.91 (0.47, 1.79)Grumme, 1995 38/175 49/195 18.7 0.83 (0.51, 1.34)Total 396/1061 296/836 100 0.91 (0.74, 1.12)(χ 2 = 15.99; df = 12; Z = 0.89)0.1 0.2 1.0 5.0 10.0Steroids better Steroids worseReproduced with permission from the BMJ Publishing Group (BMJ 1997;314:1855–9).pooled effect, some researchers prefer to use wider CIs (often 99%) for thepooled estimate on the grounds that evidence regarding treatment benefit shouldbe more convincing when combining the results of multiple studies. CIs for thepooled estimate are much narrower than for the individual studies because morepatients contribute to the overall estimate, and consequently it is more precise.Forest plots allow us to review at a glance a large collection of data testing thesame relationship. They allow visual assessment of heterogeneity between groupsby comparing the results of each study to see whether there is a single underlyingeffect, or a distribution of effects. If the results of the studies differ greatly, andthe CIs for all of the studies do not overlap, then it might not be appropriate tocombine the results. Although there is no statistical solution to this, heterogeneitybetween studies should not be seen simply as a problem for meta-analysis – it alsoprovides an opportunity for examining why treatment effects differ in differentcircumstances [6].422