[R] Creating subsets with factors

[Sven Garbade]

Hi all,

I don't understand the following output. I've created a data subset from
a data frame by

> p1.sub <- subset(p1.dat, vp!="p1")

this is ok. But 

> attach(p1.sub)
> vp
 [1] p1ab p1ab p1ab p1ab p1ab p1br p1br p1br p1br p1br p1kf p1kf p1kf
p1kf p1kf
[16] p1mg p1mg p1mg p1mg p1mg p1mw p1mw p1mw p1mw p1mw
Levels:  p1 p1ab p1br p1kf p1mg p1mw 

shows me that the factor vp has 6 levels instead of 5? 5 should be the
correct number of levels, because p1 isn't in the data subset.

Thanks, Sven
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