[R] Package or function for selecting matched pairs?

Ista Zahn izahn at psych.rochester.edu
Wed Feb 17 14:26:57 CET 2010

Hi all,
I am designing a study in which I am selecting a subset of college
courses to be randomly assigned to one of two conditions. I would like
to create matched pairs of courses, and then randomly assign them to
condition within each pair. I would like to identify, for each course,
the one that best matches it, and quantify how well it matches. Here
is a much simpler data set for purposes of illustration:

ED <- data.frame(course = letters[1:5], level=factor(c(100, 100, 200,
300, 200)), size = c(44, 12, 23, 124, 30), rating = c(4,5,5,3,5))

> >   course level size rating
1      a   100   44      4
2      b   100   12      5
3      c   200   23      5
4      d   300  124      3
5      e   200   30      5

Basically I want a function that tells me that courses c and e "match"
so that I can treat them as a pair and randomly assign them to
condition. I've looked at the matching and MatchIt packages, but they
seem to need to know in advance which course is in the treatment
condition and which is in the control condition. I'll be grateful for
any suggestions.

Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology

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