Naimi Ashley I
Am J Epidemiol. 2015 Jul 15;182(2):109-14. doi: 10.1093/aje/kwv060. Epub 2015 May 5.
Epidemiologists are increasingly using natural effects for applied mediation analyses, yet 1 key identifying assumption is unintuitive and subject to some controversy. In this issue of the Journal, Jiang and VanderWeele (Am J Epidemiol. 2015;182(2):105-108) formalize the conditions under which the difference method can be used to estimate natural indirect effects. In this commentary, I discuss implications of the controversial "cross-worlds" independence assumption needed to identify natural effects. I argue that with a binary mediator, a simple modification of the authors' approach will provide bounds for natural direct and indirect effect estimates that better reflect the capacity of the available data to support empirical statements on the presence of mediated effects. I discuss complications encountered when odds ratios are used to decompose effects, as well as the implications of incorrectly assuming the absence of exposure-induced mediator-outcome confounders. I note that the former problem can be entirely resolved using collapsible measures of effect, such as risk ratios. In the Appendix, I use previous derivations for natural direct effect bounds on the risk difference scale to provide bounds on the odds ratio scale that accommodate 1) uncertainty due to the cross-world independence assumption and 2) uncertainty due to the cross-world independence assumption and the presence of exposure-induced mediator-outcome confounders.
流行病学家越来越多地将自然效应用于应用中介分析，但有一个关键的识别假设并不直观且存在一些争议。在本期《杂志》中，Jiang和VanderWeele（《美国流行病学杂志》。2015年；182(2):105 - 108）正式阐述了可使用差异法估计自然间接效应的条件。在这篇评论中，我讨论了识别自然效应所需的有争议的“跨世界”独立性假设的影响。我认为，对于二元中介变量，对作者方法进行简单修改将为自然直接效应和间接效应估计提供界限，能更好地反映现有数据支持关于中介效应存在的实证陈述的能力。我讨论了使用比值比分解效应时遇到的复杂性，以及错误假设不存在暴露诱导的中介 - 结局混杂因素的影响。我指出，前一个问题可以使用可折叠的效应量度（如风险比）完全解决。在附录中，我使用先前在风险差尺度上对自然直接效应界限的推导，来提供比值比尺度上的界限，以适应1）由于跨世界独立性假设导致的不确定性，以及2）由于跨世界独立性假设和暴露诱导的中介 - 结局混杂因素的存在导致的不确定性。
