Hi Kate,
I'm thinking that the reason why you see this behavior is because you
use exact matching (method = "exact"). This means that an observation
which cannot find an exact match will be discarded even if you set discard
= "none". Let us know if the problem persists even with non-exact
matching.
Many thanks,
Kosuke
-----------------------------------------------------
Kosuke Imai Office: Corwin Hall 041
Assistant Professor Phone: 609-258-6601
Department of Politics eFax: 973-556-1929
Princeton University Email: kimai(a)Princeton.Edu
Princeton, NJ 08544-1012
http://imai.princeton.edu
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On Mon, 12 Jun 2006, Kate Crespi wrote:
Dear Prof. Imai,
I was very happy to discover the MatchIt package in R and would like to us
it in my research. However, I've come across what might be a bug. I have
found that even when the discard="none" option is specified, the matchit
function still discards outlying observations. Below is an example. I'm
not sure whether the program is intended to work this way, so I thought to
bring it to your attention. Users of the package would most likely find it
more useful if they could control whether or not observations are discarded.
testdata <- matrix(runif(150,0,0.1), nrow=10, ncol=15)
testdata <- rbind(runif(15,0.9,1), testdata, testdata) #add an outlie
tx <- c(rep(0,11),rep(1,10))
testdata <- cbind(tx, testdata)
colnames(testdata) <- c("tx", 1:15)
testdata <- data.frame(testdata)
mtest <- matchit(tx~X1+X2+X3+X4+X5+X6+X7+X8+X9+X10+X11+X12+X13+X14+X15,
testdata, method="exact",
discard="none")
summary(mtest)
Sincerely,
Kate Crespi
Catherine M. Crespi, Ph.D.
Postdoctoral Fellow
Department of Biostatistics
University of California Los Angeles
ccrespi(a)ucla.edu
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