  Long-term outcomes of experimental evaluations are necessarily observed after
long delays. We develop semiparametric methods for combining the short-term
outcomes of experiments with observational measurements of short-term and
long-term outcomes, in order to estimate long-term treatment effects. We
characterize semiparametric efficiency bounds for various instances of this
problem. These calculations facilitate the construction of several estimators.
We analyze the finite-sample performance of these estimators with a simulation
calibrated to data from an evaluation of the long-term effects of a poverty
alleviation program.
