Coarsened exact matching, causal inference, sas, sas iml 1 introduction an important branch of the existing literature on causal inference is represented by observational studies that often are the only available alternatives to the lack of randomization, especially in nonu. The stata blog exact matching on discrete covariates is the. Example sas code for matching two samples is provided, as well as guidance for expanding the match to three or more groups. Coarsened exact matching cem on panel data statalist. Exact matching dear statalist members, i am trying to run a coarsened exact matching to create comparable pair of individuals within my sample. Gary king of harvard university offers a different take on matching procedures by means of a new technique known as coarsened exact matching cem. Stefano iacus, gary king, giuseppe porro this program is designed to improve the estimation of causal effects via an extremely powerful method of matching that is widely applicable and exceptionally easy to understand and use if you understand how to draw a histogram, you will understand this. Study subjects were matched according to characteristics not assessed by laced, including duration of surgery, wound class, body mass. The program implements the coarsened exact matching cem algorithm described in. This program is designed to improve causal inference via a method of matching that is widely applicable in observational data and easy to understand and use if you understand how to draw a. The goal of the original study was to estimate the effect of a job training program the variable treated on real earnings in 1978 the variable re78. Specifically, we use onetoone coarsened exact matching cem to match pre tax hike rides with analogous post tax hike rides provided by the same driver berta et al. A random number is assigned to each record for use in the examples below. I have eliminated nas and am running matching on polity2 7 treatment.
Iacus, gary king, and giuseppe porro, matching for causal inference without balance checking. Coarsened exact matching cem a simple and ancient method of causal inference, with surprisingly powerful properties preprocessx, t with cem. Comparing regression, propensity matching and coarsened exact matching in healthcare observational studies using sas. The data step reads the data and creates two sas dat asets out of the raw data, one containing the study cases and the other, the control cases. Using sas software to perform a casecontrol match on propensity score in an. Coarsened exact matching more info with stefano iacus, giuseppe porro and gary king. Coarsened exact matching coarsened exact matching cem is a monotonoic imbalance bounding mib matching method which means that the balance between the treated and control groups is chosen by the user ex ante rather than discovered through the usual laborious process of checking after the fact and repeatedly reestimating, and so that. Cem may be used alone or in combination with any existing matching method. The program implements the coarsened exact matching cem algorithm.
Matching and weighting methods for causal inference. Iacus description implementation of the coarsened exact matching algorithm. Optimal matching selects all control units that match each treated unit by minimizing the total absolute difference in propensity score across all matches. Aug 16, 2016 exact matching on discrete covariates and ra with fully interacted discrete covariates perform the same nonparametric estimation. Apr 19, 2017 a member is represented by properties coarsened to discrete values using a coarsening or binning strategy.
Cem for sas, by stefano verzillo, paolo berta, and matteo bossi. Optimal matching selects all matches simultaneously and without replacement. This option will delete some data that matched well, and thus likely increase the variance, but it means that subsequent. Exact match on age and race in this simple example, the subjects are matche d on age and race. Coarsened exact matching and multivariate matching methods that are monotonic imbalance bounding. Genetic matching contents coarsened exact matching coarsened exact matching cem is a monotonoic imbalance bounding mib matching method which means that the balance between the treated and control groups is chosen by the user ex ante rather than discovered through the usual laborious process of checking after the fact and. Thus each member is given a bin signature that will be used to exactly match other. In this paper we introduce %cem, a macro package allowing researchers to automatically perform coarsened exact matching cem in sas environment.
Coarsened exact matching was used to assess the predictive capacity of the laced index among all cardiac surgery cases over a 2year period 20162018 at 1 health system n 4001. I am trying to run coarsened exact matching on a dataset of 6549 observations and 94 variables. Whether or not another method of matching is applied, one must compute the causal e ect, either by a weighted di erence in means in y among the treated and control units, or with the application of a statistical model. Coarsened exact matching explained devmotivation medium. How to get coarsened exact matching cem do an exact match. Whether or not another method of matching is applied, one must compute the causal e. Hth david sent with verizon mobile email original message from. Sas visual statistics create and modify predictive models faster than ever using a visual interface and inmemory processing. Two roads diverged in a narrow datasetwhen coarsened. Advantages of cem relative to propensity matching are described by iacus et al. Basics of matching general strategy of matching 1 determine the variables you want to match on. It is assumed the reader has a working knowledge of basic research terminology and basic sas coding, and some minor familiarity with sas macro functions. Coarsened exact matching is faster, is easier to use and understand, requires fewer assumptions, is more easily automated, and possesses more. The matching rules are tight in the beginning and, if necessary, are systematically relaxed to allow for more matches, up to a pre determined coarsening limit.
Differences in matched units comparing available software 1. T variable of treatment x cofounders z variable with exact matching y output because i know that there are very important variables in education, i want to do exact matching in some variables z1 z2 z3 so i estimate the propensity score and i do exact matching with a tip. Software for coarsened exact matching cran r project. How to use cem for spss in order to demonstrate the syntax of cem in spss, you can download an example dataset of a job training program.
Comparing the matching properties of coarsened exact matching, propensity score matching, and genetic matching in a nationwide data and a simulation experiment by shanshan qin under the direction of karen samuelsen abstract sample matching is one statistical technique that can be applied to observational data to. Thus, we want to run cem to remove imbalances on the treated variable. In coarsened exact matching, users temporarily coarsen their data, exact match on these coarsened data, and then. Its important to match on any potential confounders as well as any imbalanced covariates. This algorithm, and its statistical properties, are described in stefano m. This program is designed to improve the estimation of causal effects via an extremely powerful method of matching that is widely applicable and exceptionally easy to understand and use if you understand how to. Coarsened exact matching cem is a relatively new causal inference. Once the covariate distributions are adjusted, standard analysis methods such as regression can.
Coarsened exact matching cem is a relatively new causal inference technique that allows the researcher to nonparametrically create a matched dataset to evaluate the effect of a treatment. Coarsened exact matching cem offers an alternative solution, which is faster and easier to understand. In coarsened exact matching, users temporarily coarsen their data, exact match on these coarsened data. Propensity score matching psm is the older, more established technique in the literature. Ricardo, the first thing you would need to do is to generate a variable that indicates whether a firm was involved in an acquisition. Coarsened exact matching is faster, is easier to use and understand, requires fewer assumptions, is more easily automated, and possesses more attractive statistical properties for many applications than do existing matching methods. Iacus, gary king, and giuseppe porro, causal inference without balance checking. Examples include estimating the effects of a training program on job performance or the effects of a government program targeted at helping particular schools. Comparing regression, propensity matching and coarsened exact.
Statistical matching offers a solution to this problem by finding statistical twins, one with and one without the treatment. A stata package for entropy balancing the estimated weights d. If the coarsened bins are set to zero width, then cem returns the exact matching solution. Cem is a package for r, stata, and spss that implements the method of coarsened exact matching.
This function transforms a typical cem matching solution to a ktok match, with k variable along strata. Dear all, while using coarsened exact matching cem, one of the striking things i noticed was that the algorithm does not offer an option which allows the user to ask for an exact match on some variables unlike other matching algorithms such as matching estimators. The most common matching approach is to match on a propensity score austin et al, 2010. Coarsened exact matching cem matches people who match on every variable, but there is flexibility in what counts as a match.
Sas software may be provided with certain thirdparty software, including but not limited to opensource software, which is. The cem package provides a stata implementation of coarsened exact matching, a new method for improving the estimation of causal effects by reducing imbalance in covariates between treated and control groups. Software for coarsened exact matching article pdf available in journal of statistical software 30i09 june 2009 with 654 reads how we measure reads. Cem is a nonparametric matching method widely used by researchers to avoid the confounding influence of pretreatment control variables to improve causal inference in quasiexperimental studies. Comparison of matching metrics and causal effects estimated from coarsened exact matching cem and propensity score matching psm effectiveness of two matching methodologies based on the qualitative and quantitative imbalance results observed in tables 1 and and2, 2, cem and psm were employed within the aforementioned sampling design. Coarsened exact matching in stata matthew blackwell1 stefano iacus2 gary king3 giuseppe porro4 february 22, 2010 1institute for quantitative social science,1737 cambridge street, harvard university, cam bridge ma 028. Age 2, gender 1, chf 0, copd 1, cancer 1 would only match a member in the other cohort with the exact same 21011 bin. This option will delete some data that matched well, and thus likely increase the variance, but it means that subsequent analyses do not require. Cem authors originally provided few software solutions for r, stata and spss. The cem command implements the coarsened exact matching algorithm in stata.
Other matching methods inherit many of the coarsened exact matching methods properties when applied to further match data preprocessed by coarsened exact matching. Feb 01, 2010 a recent advance, coarsened exact matching cem, can be used to do exact matching on broader ranges of the variables. The most common matching technique, propensity score matching, however, is slow and difficult to apply. Overview this program is designed to improve causal inference via a method of matching that is widely applicable in observational data and easy to understand. Cem via the dialog menu in order to demonstrate the syntax of cem in spss, you can download an example dataset of a job training program. The % cem macro integrates the already available software alternatives introducing a completely automated coarsened exact matching macro for sas users. How to get coarsened exact matching cem do an exact. Matthew blackwell, stefano iacus, gary king, and giuseppe porro. Both the matching strategy including some standard coarsening options and the associated l1 multivariate imbalance measure are provided.
For instance, adding one continuous variable to a dataset e ectively kills exact matching since two observations. Sas code for this approach, which can be adapted to suit a wide variety of matching needs, is located in appendix a. This algorithm, and its statistical properties, are described in iacus, king, and porro 2008. Propensity score matching is used when a group of subjects receive a treatment and wed like to compare their outcomes with the outcomes of a control group. Multivariate and propensity score matching software with automated balance. Cem improves causal inferences and reduces model dependence by making observations more comparable. Fuzzy matching programming techniques using sas software. Comparing regression, propensity matching and coarsened. Using sas software to perform a casecontrol match on propensity score in an observational study. However, some researchers have more recently advocated coarsened exact matching cem. Sas university edition get the analytical skills you need to secure your future with free sas statistical analysis software for students and professors. Coarsened exact matching in stata matthew blackwell.
Reducing bias in a propensity score matchedpair sample using greedy matching techniques. Using sas software to perform a casecontrol match on propensity score in. Collapsing categories or cutting up discrete covariates performs the same function as a bandwidth in nonparametric kernel regression. Exploring robust methods for evaluating treatment and. For example, if you used matching with the psmatch procedure, a simple univariate test or analysis might be suf. Since we are trying to match entries that do not have an exact match, we can save processing time by immediately. Fuzzy matching programming techniques using sas software, continued sgf 2018 page 3 step 1. Expanding to three samples in some cases, it may be necessary to match three or more samples for analysis. Cem is a nonparametric matching method to avoid the confounding influence of pretreatment control variables directly improving causal inference in quasiexperimental stud ies. A recent advance, coarsened exact matching cem, can be used to do exact matching on broader ranges of the variables.
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