Hi there,

I'm wondering what Zelig in the following situation (code below) actually does. Is this considered as a so called regression adjustment after the propensity score matching?

library(MatchIt)
library(Zelig)
data(lalonde)

re78 represents the outcome variable


1. With Zelig


m.out <- matchit(treat ~ age + educ + black + hispan + married + nodegree + re74 + re75, data = lalonde)

z.out <- zelig(re78 ~ distance, data = match.data(m.out, "control"), model = "ls")
x.out <- setx(z.out, fn = NULL, data = match.data(m.out, "treat"), cond = TRUE)
s.out <- sim(z.out, x = x.out)
summary(s.out)

2. Without Zelig - only matching

T.output <- match.data(m.out, group="treat")$re78
C.output <- match.data(m.out, group="control")$re78
ATT <- mean(T.output)-mean(C.output)
ATT

ATT between 1. and 2. is quite different.

Hope to hear some thoughts regarding this.

Many thanks!

Ana
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