We thank Robins et al. (1) for their thoughtful commentary on our article (2). Before responding, we wish to point out that they altered our notation slightly, using m rather than j to refer to the baseline time point.Robins et al. distinguish between two types of history-adjusted marginal structural models (HA-MSMs), those that consider an outcome at a fixed time point ("group 1," or those with multiple baseline time points for a single time-point-specific outcome) and those that consider a moving outcome ("group 2," or those that still use multiple baseline time points but use no more than a single baseline for each time-point-specific outcome). They raise the issue of model incompatibility and make the point that "only MSMs in group 1 can be incompatible and thus lead to logical inconsistencies" (1, p. 996). We concur with their discussion of the pitfalls that can be involved in modeling group 1 HA-MSMs and appreciate their emphasis on this issue
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