The term matching refers to the procedure of finding for a sample unit other units in the sample that are closest in terms of observable characteristics. The units selected are usually referred to as matches, and after repeating this procedure for all units (or a subgroup of them), the resulting subsample of units is called the matched sample. This idea is typically implemented across subgroups of a given sample, that is, for each unit in one subgroup, matches are found among units of another subgroup. A matching procedure requires defining a notion of distance, selecting the number of matches to be found, and deciding whether units will be used multiple times as a potential match. In applications, matching is commonly used as a preliminary step ...