In matchit()
, setting method = "exact"
performs exact matching.
With exact matching, a complete cross of the covariates is used to form
subclasses defined by each combination of the covariate levels. Any subclass
that doesn't contain both treated and control units is discarded, leaving
only subclasses containing treatment and control units that are exactly
equal on the included covariates. The benefits of exact matching are that
confounding due to the covariates included is completely eliminated,
regardless of the functional form of the treatment or outcome models. The
problem is that typically many units will be discarded, sometimes
dramatically reducing precision and changing the target population of
inference. To use exact matching in combination with another matching method
(i.e., to exact match on some covariates and some other form of matching on
others), use the exact
argument with that method.
This page details the allowable arguments with method = "exact"
. See
matchit()
for an explanation of what each argument means in a general
context and how it can be specified.
Below is how matchit()
is used for exact matching:
matchit(formula,
data = NULL,
method = "exact",
estimand = "ATT",
s.weights = NULL,
verbose = FALSE,
...)
a two-sided formula object containing the treatment and covariates to be used in creating the subclasses defined by a full cross of the covariate levels.
a data frame containing the variables named in formula
.
If not found in data
, the variables will be sought in the
environment.
set here to "exact"
.
a string containing the desired estimand. Allowable options
include "ATT"
, "ATC"
, and "ATE"
. The estimand controls
how the weights are computed; see the Computing Weights section at
matchit()
for details.
the variable containing sampling weights to be incorporated into balance statistics. These weights do not affect the matching process.
logical
; whether information about the matching
process should be printed to the console.
ignored.
The arguments distance
(and related arguments), exact
, mahvars
, discard
(and related arguments), replace
, m.order
, caliper
(and related arguments), and ratio
are ignored with a warning.
All outputs described in matchit()
are returned with
method = "exact"
except for match.matrix
. This is because
matching strata are not indexed by treated units as they are in some other
forms of matching. include.obj
is ignored.
In a manuscript, you don't need to cite another package when
using method = "exact"
because the matching is performed completely
within MatchIt. For example, a sentence might read:
Exact matching was performed using the MatchIt package (Ho, Imai, King, & Stuart, 2011) in R.
matchit()
for a detailed explanation of the inputs and outputs of
a call to matchit()
. The exact
argument can be used with other
methods to perform exact matching in combination with other matching
methods.
method_cem for coarsened exact matching, which performs exact matching on coarsened versions of the covariates.
data("lalonde")
# Exact matching on age, race, married, and educ
m.out1 <- matchit(treat ~ age + race + married + educ, data = lalonde,
method = "exact")
m.out1
summary(m.out1)
Run the code above in your browser using DataLab