Hello all,
When using the matchit-function for full matching, the results differ by the order of the input dataframe. That is, if the order of the data is changed, results change, too. This is surprising, because in my understanding, the optimal full algorithm should yield only one single best solution.
Am I missing something or is this an error?
Similar differences occur with the optimal algorithm.
Below you find a reproducible example. In my understanding, subclasses should be identical for the two data sets, which they are not. Thank you very much for your help!
Kai
# create data
set.seed(42)
nr <- c(1:100)
x1 <- rnorm(100, mean=50, sd=20)
x2 <- c(rep("a", 20),rep("b", 60), rep("c", 20))
x3 <- rnorm(100, mean=230, sd=2)
outcome <- rnorm(100, mean=500, sd=20)
group <- c(rep(0, 50),rep(1, 50))
df <- data.frame(x1=x1, x2=x2, outcome=outcome, group=group, row.names=nr, nr=nr)
df_neworder <- df[order(outcome),] # re-order data.frame
# perform matching
model_oldorder <- matchit(group~x1, data=df, method="full", distance ="logit")
model_neworder <- matchit(group~x1, data=df_neworder, method="full", distance ="logit")
# store matching results
matcheddata_oldorder <- match.data(model_oldorder, distance="pscore")
matcheddata_neworder <- match.data(model_neworder, distance="pscore")
# Results based on original data.frame
head(matcheddata_oldorder[order(nr),], 10)
x1 x2 outcome group nr pscore weights subclass
1 69.773776 a 489.1769 0 1 0.5409943 1.0 27
2 63.949637 a 529.2733 0 2 0.5283582 1.0 32
3 52.217666 a 526.7928 0 3 0.5028106 0.5 17
4 48.936397 a 492.9255 0 4 0.4956569 1.0 9
5 36.501507 a 512.9301 0 5 0.4685876 1.0 16
# Results based on re-ordered data.frame
head(matcheddata_neworder[order(matcheddata_neworder$nr),], 10)
x1 x2 outcome group nr pscore weights subclass
1 69.773776 a 489.1769 0 1 0.5409943 1.0 25
2 63.949637 a 529.2733 0 2 0.5283582 1.0 31
3 52.217666 a 526.7928 0 3 0.5028106 0.5 15
4 48.936397 a 492.9255 0 4 0.4956569 1.0 7
5 36.501507 a 512.9301 0 5 0.4685876 2.0 14
Kai Widdecke, M. Sc.
Universität Hamburg
