Comments (2)
Levels of a factor are dropped in a regression when there are no units with those values or when they are linearly dependent on other variables (including levels of the same factor). What likely happened is that the matching changed the distribution of the covariates to yield one of the above phenomena. For example, it may be that after matching, all of the observations for a given breeder have the same region, and vice-versa, so that that level of breeder is redundant after including region in the model. You should examine the match.data()
output to see if a phenomenon of this type is occurring.
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Thanks, @ngreifer. I'll check and will let you know.
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Related Issues (20)
- optmatch missing from CRAN HOT 2
- Memory allocation problem with method="full" in matchit : Error: (from optmatch) result would exceed 2^31-1 bytes
- weight to initial variable to achieve 100% matching for certain variable
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- cardinality matching with exact match HOT 2
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