How can time-varying confounding affect causal inference in longitudinal studies?
Instruction: Explain the concept of time-varying confounding and provide an example of how it could affect the results of a longitudinal study.
Context: Explain time-varying confounding, especially when earlier treatment affects a later confounder. Discuss why ordinary adjustment can fail and when longitudinal causal methods are justified.
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A time-varying confounder changes across follow-up and predicts both later treatment and the outcome. The particularly difficult case is treatment–confounder feedback: earlier treatment also changes that later confounder.
In a hypothetical training study, early training A0 improves skill L1; skill then influences who receives later training A1 and final performance Y...