  While many areas of machine learning have benefited from the increasing
availability of large and varied datasets, the benefit to causal inference has
been limited given the strong assumptions needed to ensure identifiability of
causal effects; these are often not satisfied in real-world datasets. For
example, many large observational datasets (e.g., case-control studies in
epidemiology, click-through data in recommender systems) suffer from selection
bias on the outcome, which makes the average treatment effect (ATE)
unidentifiable. We propose a general algorithm to estimate causal effects from
\emph{multiple} data sources, where the ATE may be identifiable only in some
datasets but not others. The key idea is to construct control variates using
the datasets in which the ATE is not identifiable. We show theoretically that
this reduces the variance of the ATE estimate. We apply this framework to
inference from observational data under an outcome selection bias, assuming
access to an auxiliary small dataset from which we can obtain a consistent
estimate of the ATE. We construct a control variate by taking the difference of
the odds ratio estimates from the two datasets. Across simulations and two case
studies with real data, we show that this control variate can significantly
reduce the variance of the ATE estimate.
