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Exact Matching

The simplest version of matching is exact. This technique matches each treated unit to all possible control units with exactly the same values on all the covariates, forming subclasses such that within each subclass all units (treatment and control) have the same covariate values. Exact matching is implemented in MATCHIT using method = "exact". Exact matching will be done on all covariates included on the right-hand side of the formula specified in the MATCHIT call. There are no additional options for exact matching. (Exact restrictions on a subset of covariates can also be specified in nearest neighbor matching; see Section 3.1.2.3.) The following example can be run by typing demo(exact) at the R prompt,

> m.out <- matchit(treat ~ educ + black + hispan, data = lalonde, 
                   method = "exact")



Gary King 2010-12-11