If one treated group is matched to one control reservoir in two different ways to produce two sets of treated-control matched pairs, then the two control groups may be entwined, in the sense that some control individuals are in both control groups. The exterior match is used to compare the two control groups.
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Paul R. Rosenbaum
Maintainer: Paul R. Rosenbaum <email@example.com>
Rosenbaum, P.R. and Silber, J.H., 2013. Using the exterior match to compare two entwined matched control groups. The American Statistician, 67(2), pp.67-75.
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