In animal behavior studies, a common goal is to investigate the causal pathways between an exposure and outcome, and a mediator that lies in between. Causal mediation analysis provides a principled approach for such studies. Although many applications involve longitudinal data, the existing causal mediation models are not directly applicable to settings where the mediators are measured on irregular time grids. In this paper, we propose a causal mediation model that accommodates longitudinal mediators on arbitrary time grids and survival outcomes simultaneously. We take a functional data analysis perspective and view longitudinal mediators as realizations of underlying smooth stochastic processes. We define causal estimands of direct and indirect effects accordingly and provide corresponding identification assumptions. We employ a functional principal component analysis approach to estimate the mediator process, and propose a Cox hazard model for the survival outcome that flexibly adjusts the mediator process. We then derive a g-computation formula to express the causal estimands using the model coefficients. The proposed method is applied to a longitudinal data set from the Amboseli Baboon Research Project to investigate the causal relationships between early adversity, adult physiological stress responses, and survival among wild female baboons. We find that adversity experienced in early life has a significant direct effect on females' life expectancy and survival probability, but find little evidence that these effects were mediated by markers of the stress response in adulthood. We further developed a sensitivity analysis method to assess the impact of potential violation to the key assumption of sequential ignorability.
翻译:在动物行为研究中,一个共同目标是调查接触和结果之间的因果途径,以及介于两者之间的调解人。由于调解分析为这类研究提供了原则性的方法。虽然许多应用都涉及纵向数据,但现有的因果调解模式并不直接适用于调解人在非正常时间网格中计量的环境。在本文件中,我们提议了一个因果调解模式,在任意的时间网和生存结果方面兼顾纵向调解人。我们从功能性数据分析角度出发,将纵向调解人视为平稳随机随机过程的实现。我们据此界定直接和间接影响的因果估计,并提供相应的识别假设。我们采用功能性主要组成部分分析方法来估计调解进程,并提出一个可灵活调整调解程序的生存结果的考克斯危险模型。我们然后用模型系数得出一个解释公式,以表达因果估计的因果估计值和生存结果。我们采用拟议方法,从安博塞利巴博诺研究项目中,进一步调查早期逆境、成人生理压力反应和野生女性生理敏感性之间的因果关系,并提供相应的相应的识别假设。我们发现,在持续性媒体周期中,对生命的早期概率分析中,这些直接判断是生命影响。我们所经历的、直接判断了生命压力的概率分析。我们发现,这些判断是生命压力对生命压力的概率的早期反应的概率分析。我们所得出的早期反应。我们所得出的概率分析,这些判断是根据生命的早期判断。