Instrumental Variable methods to target Hypothetical Estimands with longitudinal repeated measures data: Application to the STEP 1 trial
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纵向重复测量数据中针对假设性目标量的工具变量方法：STEP 1 试验应用
- 研究问题
- 存在持续治疗不依从时，如何利用重复体重测量估计全员依从的假设性治疗效应，并区分仅药物组与两组均依从的目标？
- 方法与思路
- 作者提出纵向工具变量方法，利用重复结局并允许依从效应随时间变化，定义两种全员依从情景的目标量并应用于 STEP 1。
- 材料陈述的贡献
- 摘要称该方法不依赖原补充分析的无混杂假设，对正值性违反较不敏感；两种目标量均提示持续且缓慢衰减的减重效应。
- 阅读关注点
- 应核对工具变量及其有效性假设、两种假设性目标量的精确定义、时变效应模型，以及不依从必然发生时的识别条件。
- 阅读边界
- 这是 arXiv 预印本，不能据此认定已同行评审或被期刊接收；摘要未说明替代识别假设，不能理解为无需假设即可校正不依从。
原文摘要
The STEP 1 randomized trial evaluated the effect of taking semaglutide vs placebo on body weight over a 68 week duration. As with any study evaluating an intervention delivered over a sustained period, non-adherence was observed. This was addressed in the original trial analysis within the Estimand Framework by viewing non-adherence as an intercurrent event. The primary analysis applied a treatment policy strategy which viewed it as an aspect of the treatment regimen, and thus made no adjustment for its presence. A supplementary analysis used a hypothetical strategy, targeting an estimand that would have been realised had all participants adhered, under the assumption that no post-baseline variables confounded adherence and change in body weight. In this paper we propose an alternative Instrumental Variable method to adjust for non-adherence which does not rely on the same `unconfoundedness' assumption and is less vulnerable to positivity violations (e.g., it can give valid results even under conditions where non-adherence is guaranteed). Unlike many previous Instrumental Variable approaches, it makes full use of the repeatedly measured outcome data, and allows for a time-varying effect of treatment adherence on a participant's weight. We show that it provides a natural vehicle for defining two distinct hypothetical estimands: the treatment effect if all participants would have adhered to semaglutide, and the treatment effect if all participants would have adhered to both semaglutide and placebo. When applied to the STEP 1 study, they both suggest a sustained, slowly decaying weight loss effect of semaglutide treatment.