Department of Clinical, Neuro and Developmental Psychology,Vrije Universiteit Amsterdam,The Netherlands.
Department of Clinical Psychology and Psychotherapy,Babes-Bolyai University,Cluj-Napoca,Romania.
Epidemiol Psychiatr Sci. 2017 Aug;26(4):364-368. doi: 10.1017/S2045796016000809. Epub 2016 Oct 28.
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).
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.
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.
We conclude that pre-post SMDs should be avoided in meta-analyses as using them probably results in biased outcomes.
目的 标准化均数差（SMD）是最常用于表示治疗效果的效应量之一。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，包括一些引用率高且有影响力的荟萃分析，使用前后 SMD，这表明在一个（治疗组）中，在基线和后测之间的差异。
在本文中，我们认为这些前后 SMD 不应在荟萃分析中使用，并描述了为什么前后 SMD 可能导致有偏结果的原因。
前后 SMD 不应使用的一个重要原因是，基线和后测的分数彼此不独立。在计算 SMD 时应使用相关值，而通常不知道该值。我们使用了一项比较认知行为疗法和抗抑郁药物的试验的“个体患者数据”荟萃分析的数据，以表明该问题可能导致 SMD 估计中的相当大的误差。前后 SMD 不应在荟萃分析中使用的另一个更重要的原因是，它们受到患者和环境的自然过程和特征的影响，而这些过程和特征无法从干预效果中辨别出来。组间 SMD 更好，因为它们控制了这些变量，并且只有当这些变量与干预效果相关时，这些变量才会影响组间 SMD。
我们的结论是，前后 SMD 不应在荟萃分析中使用，因为使用它们可能会导致有偏结果。
