Hello,
I am very interested in applying 'MatchIt' to a health study, but I noticed that each time when I ran MatchIt() using the nearest neighbor matching with a caliper distance of 0.2 and with replacement, the resulting matched set yielded a slightly different number of matched treated and control subjects, despite the same PS model.
I thought that the matched set is invariable, but maybe my intuitions about PS matching are not very good.
To illustrate this, I attached a R code below:
====
library(MatchIt)
m.out <- matchit(treat ~ re74 + re75 + educ + factor(race) + age, data = lalonde, method = "nearest", ratio=3, caliper=0.2, replacement=TRUE)
m.out
# 1
#Sample sizes:
# Control Treated
#All 429 185
#Matched 178 115
#Unmatched 251 70
#Discarded 0 0
# 2
#Sample sizes:
# Control Treated
#All 429 185
#Matched 178 116
#Unmatched 251 69
#Discarded 0 0
====
Thank you,
Hong
