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Possible Duplicate:
dropping factor levels in a subsetted data frame in R
I have subsetted away observations with a certain factor level. When checking whether this has been done with summary() the levels were still listed, but with zero observations. Shouldn’t they disappear during the subsetting?
Subsetting doesn’t drop empty levels. Why this is the case is that it is a feature. Think of it as your factor levels determine the possible/potential categories of a thing. If you only take a subset of these things, the possible categories of thing don’t change, your subset just doesn’t contain any of them.
If you want to drop these empty levels, see
?droplevels.