Instruction: Explain the concept of time-varying confounding and provide an example of how it could affect the results of a longitudinal study.
Context: Time-varying confounding occurs when confounder values change over time and are themselves influenced by past exposure or outcomes. This question tests the candidate's understanding of more complex aspects of causal inference in studies that track participants over time. An effective answer would discuss how this influences the interpretation of results and what methods can be employed to address this issue, such as time-varying covariates in a multivariate model.