  Survey respondents may give untruthful answers to sensitive questions when
asked directly. In recent years, researchers have turned to the list experiment
(also known as the item count technique) to overcome this difficulty. While
list experiments may be less prone to bias than direct questioning, list
experiments are also more susceptible to sampling variability. We show that
researchers do not have to abandon direct questioning altogether in order to
gain the advantages of list experimentation. We develop a nonparametric
estimator of the prevalence of sensitive behaviors that combines list
experimentation and direct questioning. We prove that this estimator is
asymptotically more efficient than the standard difference-in-means estimator,
and we provide a basis for inference using Wald-type confidence intervals.
Additionally, leveraging information from the direct questioning, we derive two
nonparametric placebo tests of the identifying assumptions for the list
experiment. We demonstrate the effectiveness of our combined estimator and
placebo tests with an original survey experiment.
