A new exposure classification system for epidemiologic research

Background Epidemiologic exposures arise through different processes, and these processes may shape confounding, selection, measurement, and the feasibility of alternative study designs. A simple framework focused on the exposure-generating process may therefore be useful during early study planning. Methods This was not a systematic review; instead, it was a focused conceptual synthesis of epidemiologic literature on intended and unintended effects, confounding by indication, well-defined interventions, bias, and causal study design. We applied the proposed dimensions to classic and contemporary exposure questions. Results We propose classifying a specified acquired exposure pathway or causal contrast along two dimensions: whether exposure initiation or control is participant/proxy-directed or externally determined, and whether the outcome under study is an intended or unintended effect. Their intersection yields four classes. The classification is conditional on the population, decision-maker, exposure contrast, outcome, and time period; it is not an intrinsic label attached to an exposure. Information bias is treated as a cross-cutting threat, and genetic characteristics are outside the framework’s scope. Conclusions The framework may help investigators anticipate threats to validity and identify feasible study designs. It is a pedagogical heuristic that complements, but does not replace, formal causal tools.