Background Overadjustment bias occurs when researchers adjust for an explanatory variable on the causal pathway from exposure to outcome, which leads to biased estimates of the causal effect of the exposure. This meta-research review aimed to examine how previous systematic reviews and meta-analyses of socio-economic inequalities in health have managed overadjustment bias. Methods We searched Medline and Embase until 16 April 2021 for systematic reviews and meta-analyses of observational studies on associations between individual-level socio-economic position and health outcomes in any population. A set of criteria were developed to examine methodological approaches to overadjustment bias adopted by included reviews (rated Yes/No/Somewhat/Unclear). Results Eighty-four reviews were eligible (47 systematic reviews, 37 meta-analyses). Regarding approaches to overadjustment, whereas 73% of the 84 reviews were rated as Yes for clearly defining exposures and outcomes, all other approaches were rated as Yes for <55% of reviews; for instance, 5% clearly defined confounders and mediators, 2% constructed causal diagrams and 35% reported adjusted variables for included studies. Whereas only 2% included overadjustment in risk of bias assessment, 54% included confounding. Of the 37 meta-analyses, 16% conducted sensitivity analyses related to overadjustment. Conclusions Our findings suggest that overadjustment bias has received insufficient consideration in systematic reviews and meta-analyses of socio-economic inequalities in health. This is a critical issue given that overadjustment bias is likely to result in biased estimates of health inequalities and accurate estimates are needed to inform public health interventions. There is a need to highlight overadjustment bias in review guidelines.

中文翻译：

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健康领域社会经济不平等的系统评价和荟萃分析中的过度调整偏差：荟萃研究范围界定综述

背景 当研究人员调整从暴露到结果的因果路径的解释变量时，就会出现过度调整偏差，从而导致对暴露因果效应的估计出现偏差。这项荟萃研究综述旨在探讨之前对健康领域社会经济不平等的系统综述和荟萃分析如何管理过度调整偏差。方法 我们检索了 Medline 和 Embase，直至 2021 年 4 月 16 日，以对任何人群中个人层面的社会经济地位与健康结果之间的关联的观察性研究进行系统回顾和荟萃分析。制定了一套标准来检查纳入的评论所采用的过度调整偏差的方法学方法（评级是/否/有些/不清楚）。结果 84 项评价合格（47 项系统评价，37 项荟萃分析）。关于过度调整的方法，虽然 84 条评论中的 73% 因明确定义暴露和结果而被评为“是”，但所有其他方法在不到 55% 的评论中被评为“是”；例如，5% 的人明确定义了混杂因素和中介因素，2% 的人构建了因果图，35% 的人报告了纳入研究的调整变量。虽然只有 2% 的人对偏倚风险评估进行了过度调整，但 54% 的人包括了混杂因素。在 37 项荟萃分析中，16% 进行了与过度调整相关的敏感性分析。结论 我们的研究结果表明，在健康领域社会经济不平等的系统评价和荟萃分析中，过度调整偏差没有得到充分考虑。这是一个关键问题，因为过度调整偏差可能会导致对健康不平等的估计有偏差，并且需要准确的估计来为公共卫生干预措施提供信息。有必要在审查指南中强调过度调整偏差。