  The measurement of the efficiency of an event selection is always an
important part of the analysis of experimental data. The statistical techniques
which are needed to determine the efficiency and its uncertainty are reviewed.
Frequentist and Bayesian approaches are illustrated, and the problem of
choosing a meaningful prior is explicitly addressed. Several practical use
cases are considered, from the problem of combining different samples to
complex situations in which non-unit weights or non-independent selections have
been used. The Bayesian approach allows to find analytical expressions which
solve even the most complicate problems, which make use of the family of Beta
distributions, the conjugate priors for the binomial sampling.
