Estimation of the causal eﬀects of time-varying exposures
In this chapter we describe methods for the estimation of the causal eﬀect of a timevarying exposure on an outcome of interest from longitudinal data collected in an observational study. The terms “exposure” and “treatment” will be used synonymously and
interchangeably. We assume a ﬁxed study population, that is, a closed cohort with a welldeﬁned, known start of follow-up date for each subject. Time will refer to time since start of follow-up, which we also refer to as time since baseline. We only consider estimation of the eﬀect of exposures occurring at or after the start of follow-up because the estimation of the eﬀects of pre-baseline exposures is not possible without making strong, untestable assumptions. We refer to the exposure received at start of follow-up as the baseline exposure. Baseline covariates refer to covariates, including pre-baseline exposure, that occur prior to the baseline exposure. We classify exposures as either ﬁxed or time-varying.