Department of Clinical Psychopharmacology and Neurotoxicology, National Institute of Mental Health and Neurosciences, Bangalore, India.
J Clin Psychiatry. 2020 Sep 22;81(5):20f13681. doi: 10.4088/JCP.20f13681.
In randomized controlled trials (RCTs), endpoint scores, or change scores representing the difference between endpoint and baseline, are values of interest. These values are compared between experimental and control groups, yielding a mean difference between the experimental and control groups for each outcome that is compared. When the mean difference values for a specified outcome, obtained from different RCTs, are all in the same unit (such as when they were all obtained using the same rating instrument), they can be pooled in meta-analysis to yield a summary estimate that is also known as a mean difference (MD). Because pooling of the mean difference from individual RCTs is done after weighting the values for precision, this pooled MD is also known as the weighted mean difference (WMD). Sometimes, different studies use different rating instruments to measure the same outcome; that is, the units of measurement for the outcome of interest are different across studies. In such cases, the mean differences from the different RCTs cannot be pooled. However, these mean differences can be divided by their respective standard deviations (SDs) to yield a statistic known as the standardized mean difference (SMD). The SD that is used as the divisor is usually either the pooled SD or the SD of the control group; in the former instance, the SMD is known as Cohen's d, and in the latter instance, as Glass' delta. SMDs of 0.2, 0.5, and 0.8 are considered small, medium, and large, respectively. SMDs can be pooled in meta-analysis because the unit is uniform across studies. This article presents and explains the different terms and concepts with the help of simple examples.
在随机对照试验（RCT）中，终点评分或代表终点与基线差异的变化评分是感兴趣的值。这些值在实验组和对照组之间进行比较，得出每个比较结果的实验组和对照组之间的平均差异。当从不同 RCT 获得的特定结果的平均差异值都在相同的单位中（例如，它们都是使用相同的评分工具获得的）时，可以在荟萃分析中汇总以得出综合估计值，也称为平均差异（MD）。由于对个体 RCT 的平均差异进行汇总是在为精度加权值之后进行的，因此汇总的 MD 也称为加权平均差异（WMD）。有时，不同的研究使用不同的评分工具来测量相同的结果；也就是说，感兴趣的结果的度量单位在不同的研究中是不同的。在这种情况下，不同 RCT 的平均差异不能汇总。然而，可以将这些平均差异除以各自的标准差（SD），以得出称为标准化平均差异（SMD）的统计量。用作除数的 SD 通常是汇总的 SD 或对照组的 SD；在前一种情况下，SMD 称为 Cohen's d，在后一种情况下，称为 Glass' delta。0.2、0.5 和 0.8 的 SMD 分别被认为是小、中和大。SMD 可以在荟萃分析中汇总，因为单位在各个研究中是统一的。本文通过简单的示例介绍和解释了不同的术语和概念。
