MatchIt: Nonparametric Preprocessing for Parametric Causal Inference 
Overview
MatchIt provides a simple and straightforward interface to various
methods of matching for covariate balance in observational studies.
Matching is one way to reduce confounding and model dependence when
estimating treatment effects. Several matching methods are available,
including nearest neighbor matching, optimal pair matching, optimal full
matching, generalized full matching, genetic matching, exact matching,
coarsened exact matching, cardinality matching, and subclassification,
some of which rely on functions from other R packages. A variety of
methods to estimate propensity scores for propensity score matching are
included. Below is an example of the use of MatchIt to perform
Mahalanobis distance matching with replacement and assess balance:
library("MatchIt")
data("lalonde", package = "MatchIt")
# 1:1 nearest neighbor matching with replacement on
# the Mahalanobis distance
m.out <- matchit(treat ~ age + educ + race + married +
nodegree + re74 + re75,
data = lalonde, distance = "mahalanobis",
replace = TRUE)Printing the MatchIt object provides details of the kind of matching
performed.
m.out#> A matchit object
#> - method: 1:1 nearest neighbor matching with replacement
#> - distance: Mahalanobis
#> - number of obs.: 614 (original), 260 (matched)
#> - target estimand: ATT
#> - covariates: age, educ, race, married, nodegree, re74, re75
We can check covariate balance for the original and matched samples
using summary():
#Checking balance before and after matching:
summary(m.out)#>
#> Call:
#> matchit(formula = treat ~ age + educ + race + married + nodegree +
#> re74 + re75, data = lalonde, distance = "mahalanobis", replace = TRUE)
#>
#> Summary of Balance for All Data:
#> Means Treated Means Control Std. Mean Diff. Var. Ratio eCDF Mean eCDF Max
#> age 25.8162 28.0303 -0.3094 0.4400 0.0813 0.1577
#> educ 10.3459 10.2354 0.0550 0.4959 0.0347 0.1114
#> raceblack 0.8432 0.2028 1.7615 . 0.6404 0.6404
#> racehispan 0.0595 0.1422 -0.3498 . 0.0827 0.0827
#> racewhite 0.0973 0.6550 -1.8819 . 0.5577 0.5577
#> married 0.1892 0.5128 -0.8263 . 0.3236 0.3236
#> nodegree 0.7081 0.5967 0.2450 . 0.1114 0.1114
#> re74 2095.5737 5619.2365 -0.7211 0.5181 0.2248 0.4470
#> re75 1532.0553 2466.4844 -0.2903 0.9563 0.1342 0.2876
#>
#> Summary of Balance for Matched Data:
#> Means Treated Means Control Std. Mean Diff. Var. Ratio eCDF Mean eCDF Max Std. Pair Dist.
#> age 25.8162 25.5622 0.0355 0.6540 0.0461 0.1838 0.4827
#> educ 10.3459 10.4270 -0.0403 1.1635 0.0077 0.0378 0.1963
#> raceblack 0.8432 0.8432 0.0000 . 0.0000 0.0000 0.0000
#> racehispan 0.0595 0.0595 0.0000 . 0.0000 0.0000 0.0000
#> racewhite 0.0973 0.0973 0.0000 . 0.0000 0.0000 0.0000
#> married 0.1892 0.1784 0.0276 . 0.0108 0.0108 0.0276
#> nodegree 0.7081 0.7081 -0.0000 . 0.0000 0.0000 0.0000
#> re74 2095.5737 1788.6941 0.0628 1.5689 0.0311 0.1730 0.2494
#> re75 1532.0553 1087.7420 0.1380 2.1220 0.0330 0.0865 0.2360
#>
#> Sample Sizes:
#> Control Treated
#> All 429. 185
#> Matched (ESS) 32.94 185
#> Matched 75. 185
#> Unmatched 354. 0
#> Discarded 0. 0
At the top is balance for the original sample. Below that is balance in the matched sample, followed by the percent reduction in imbalance and the sample sizes before and after matching. Smaller values for the balance statistics indicate better balance. (In this case, good balance was not achieved and other matching methods should be tried). We can plot the standardized mean differences in a Love plot for a clean, visual display of balance across the sample:
#Plot balance
plot(summary(m.out))
Although much has been written about matching theory, most of the theory
relied upon in MatchIt is described well in Ho, Imai, King, and
Stuart (2007) and Stuart
(2010). The Journal of Statistical
Software article for MatchIt can be accessed
here, though note that some
options have changed, so the MatchIt reference pages and included
vignettes should be used for understanding the functions and methods
available. Further references for individual methods are present in
their respective help pages. The MatchIt
website provides access to
vignettes and documentation files.
Citing MatchIt
Please cite MatchIt when using it for analysis presented in
publications, which you can do by citing the Journal of Statistical
Software article below:
Ho, D. E., Imai, K., King, G., & Stuart, E. A. (2011). MatchIt: Nonparametric Preprocessing for Parametric Causal Inference. Journal of Statistical Software, 42(8). doi:10.18637/jss.v042.i08
This citation can also be accessed using citation("MatchIt") in R. For
reproducibility purposes, it is also important to include the version
number for the version used.