Suharto,
  I think that the key is to read the definition of exclude in the
Arguments section:

a vector of values to be excluded when forming the set of levels. This
should be of the same type as x, and will be coerced if necessary.

  Because the levels already exist for x as a factor, they are not formed
or revised, except to drop unused levels in the case where exclude=NULL (or
the default value). To drop level a from x use:

factor(as.character(x), exclude="a")
  or, on creation:

x <- factor(c("a", "b"), exclude="a")

Dave


On Wed, Dec 5, 2012 at 11:39 PM, Suharto Anggono Suharto Anggono <
suharto_anggono@yahoo.com> wrote:

> factor(x, exclude=factor("a", levels=c("a","b")))

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