  Causal inferences from a randomized controlled trial (RCT) may not pertain to
a target population where some effect modifiers have a different distribution.
Prior work studies generalizing the results of a trial to a target population
with no outcome but covariate data available. We show how the limited size of
trials makes generalization a statistically infeasible task, as it requires
estimating complex nuisance functions. We develop generalization algorithms
that supplement the trial data with a prediction model learned from an
additional observational study (OS), without making any assumptions on the OS.
We theoretically and empirically show that our methods facilitate better
generalization when the OS is high-quality, and remain robust when it is not,
and e.g., have unmeasured confounding.
