Outcomes of great importance to patients, such as quality of life and severity of anxiety or depression, are often measured using different scales. When an outcome is measured using several scales across trials, it requires standardization to be pooled in a meta-analysis.
Common methods of standardization include using the standardized mean difference (SMD), converting continuous data to binary relative and absolute association measures, the minimally important difference (MID), the ratio of means, and transforming standardized effects back to original scales. The underlying assumption in all these methods is that the different scales measure the same construct. This paper, in BMJ, describes these methods and suggests approaches for interpretation.
Reference: Murad Mohammad Hassan, Wang Zhen, Chu Haitao, Lin Lifeng. When continuous outcomes are measured using different scales: guide for meta-analysis and interpretation BMJ 2019; 364 :k4817. https://www.bmj.com/content/364/bmj.k4817
