Pre-post effect sizes should be avoided in meta-analyses
Pre-post effect sizes should be avoided in meta-analyses
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DOI：
10.1017/s2045796016000809
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发表时间：
2017-08-01
影响因子：
8.1
通讯作者：
Twisk, J.
中科院分区：
文献类型：
--
作者：
Cuijpers, P.;Weitz, E.;Twisk, J.
目的标准化平均差（SMD）是用来表示治疗效果的最常用的效应量之一。它表示治疗结束后治疗组与对照组之间的差异，以标准差表示。一些荟萃分析，包括一些高引用和有影响力的分析，使用了前后SMD，表明一个（治疗组）内基线和后测试之间的差异。在本文中，我们认为这些后贴片应该在荟萃分析中避免，我们描述了为什么后贴片会导致有偏倚的结果。结果避免前后贴片的一个重要原因是基线和后测的分数不是相互独立的。在计算SMD时应该使用相关值，而这个值通常是未知的。我们使用了比较认知行为疗法和抗抑郁药物的“个体患者数据”荟萃分析试验的数据，以表明这个问题可能导致smd估计的相当大的错误。另一个更重要的原因是，在荟萃分析中应该避免前后smd，因为它们受到自然过程、患者特征和环境的影响，而这些无法从干预的效果中辨别出来。组间SMD要好得多，因为它们控制了这些变量，而这些变量只有在与干预效果有关时才会影响组间SMD。我们的结论是，在荟萃分析中应该避免使用前后smd，因为使用它们可能会导致结果偏倚。
Aims The standardised mean difference (SMD) is one of the most used effect sizes to indicate the effects of treatments. It indicates the difference between a treatment and comparison group after treatment has ended, in terms of standard deviations. Some meta-analyses, including several highly cited and influential ones, use the pre-post SMD, indicating the difference between baseline and post-test within one (treatment group).Methods In this paper, we argue that these pre-post SMDs should be avoided in meta-analyses and we describe the arguments why pre-post SMDs can result in biased outcomes.Results One important reason why pre-post SMDs should be avoided is that the scores on baseline and post-test are not independent of each other. The value for the correlation should be used in the calculation of the SMD, while this value is typically not known. We used data from an 'individual patient data' meta-analysis of trials comparing cognitive behaviour therapy and anti-depressive medication, to show that this problem can lead to considerable errors in the estimation of the SMDs. Another even more important reason why pre-post SMDs should be avoided in meta-analyses is that they are influenced by natural processes and characteristics of the patients and settings, and these cannot be discerned from the effects of the intervention. Between-group SMDs are much better because they control for such variables and these variables only affect the between group SMD when they are related to the effects of the intervention.Conclusions We conclude that pre-post SMDs should be avoided in meta-analyses as using them probably results in biased outcomes.