Question about calipers
by Horst, Jeanne - horstsj
Hello everyone,
First, thank you so much for your beautiful work on the MatchIt package. I use MatchIt regularly in research and am also teaching a course on Quasi-Experimental Designs and Propensity Score matching this semester. Your work has been of great help.
Second, I have a question about caliper matching. In the current literature (e.g., Austin's work) are recommendations for calipers that are 0.2 sd of the logit. In the past, I computed the standard deviation of the propensity score (m.out$distance in the probability metric). Recently, based upon the literature recommendations, I have been begun computing the standard deviation on the logit of the propensity score (logit metric, rather than probability metric), which makes more sense, given that the logit is linear.
However, I wondered what distance metric the MatchIt package is using to conduct the matching process (e.g., Nearest Neighbor). Is it being conducted on the propensity score metric or the logit metric? The standard deviations of the propensity score can be quite different from the standard deviation of the logit. Moreover, I struggle with whether it makes sense to even compute a standard deviation of the propensity score in the probability metric.
Any guidance, would be helpful. I also noticed that the matchit code offers the option to simply state the caliper distance -- is that on the metric of the logit or the propensity score (probability metric)?
Third, when conducting Mahalanobis distance, is the matrix of Mahalanobis distances stored anywhere accessible? Depending on sample size, the matrix would have to be huge, so I'm not sure what I would even do with it, other than illustrate it to my class. (I have not peeked into the source code to check.)
Thank you for any help or guidance you can offer! My students also thank you.
Best,
Jeanne
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S. Jeanne Horst, PhD
Center for Assessment and Research Studies
Associate Professor, Department of Graduate Psychology
James Madison University
1122 Lakeview Hall; 298 Port Republic Road; MSC 6806
Harrisonburg, VA 22807
Office Phone: (540) 568-7103
http://www.psyc.jmu.edu/assessment/people/horst.html
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