  When constructing a model to estimate the causal effect of a treatment, it is
necessary to control for other factors which may have confounding effects.
Because the ignorability assumption is not testable, however, it is usually
unclear which minimal set of controls is appropriate -- as is their appropriate
functional form in the model -- and effect estimation can be sensitive to these
choices. A common approach in this case is to fit several models, each with a
different control specification (under the assumption that the available
controls are sufficient but possibly not all necessary to deconfound the
treatment effect), but it is difficult to reconcile inference for the treatment
effect under the multiple resulting posterior distributions. Therefore we
propose a two-stage approach to measure the sensitivity of effect estimation
with respect to control specification. In the first stage, a model is fit with
all available controls using a prior carefully selected to adjust for
confounding. In the second stage, posterior distributions are calculated for
the treatment effect under submodels of nested sets of controls using projected
posteriors under the full model, providing valid Bayesian inference. We
demonstrate how our approach can be used to detect influential confounders in a
dataset, and apply it in a sensitivity analysis of an observational study
measuring the effect of legalized abortion on crime rates.
