Fig. 3. Graphical representation of scenarios involving longitudinal data and imperfect measurement of variables, for a variant j.

Different longitudinal data scenarios are shown in (A) through (D), with $X_1$ and $X_2$ corresponding to an earlier and later measurement of the exposure. Causal effects on the outcome occur either (A) at the earlier time point, (B) the later time point, or (C) at both time points, with in (D) an additional scenario where variant $j$ directly affects only the later measurement of the exposure and not the earlier one. In (E) is shown a scenario where the observed exposure $X_{obs}$ does not fully represent the causally relevant instance $X$ of the exposure, and the same in (F) for an observed outcome $Y_{obs}$ that does not fully represent the causally relevant outcome $Y$.