  Oversubscribed treatments are often allocated using randomized waiting lists.
Applicants are ranked randomly, and treatment offers are made following that
ranking until all seats are filled. To estimate causal effects, researchers
often compare applicants getting and not getting an offer. We show that those
two groups are not statistically comparable. Therefore, the estimator arising
from that comparison is inconsistent. We propose a new estimator, and show that
it is consistent. Finally, we revisit an application, and we show that using
our estimator can lead to sizably different results from those obtained using
the commonly used estimator.
