actually, you can think of exact matching as a limiting case of MH matching where you only match units if MH distance is zero. but note that if one unit has the same MH distance with two different units, this does not necessarily mean that those two units have the same covariate values. hope this helps, Kosuke -- Department of Politics Princeton University http://imai.princeton.edu On Wed, 9 Mar 2011, Kosuke Imai wrote: -- MatchIt mailing list served by Harvard-MIT Data Center List Address: matchit(a)lists.gking.harvard.edu Subscribe/Unsubscribe: http://lists.gking.harvard.edu/?info=matchit MatchIt Software and Documentation: http://gking.harvard.edu/matchit/ Browse/Search List Archive: http://lists.hmdc.harvard.edu/lists/matchit/