Re moving levels of a factor

Henrik Wahren wrote:

How can one or more levels be removed from a factor of a data frame. There
was a similar post on how to do this when a factor meets some criterion
(e.g. <= 2), but I can?t seem to get that solution to work.
Here, I simply want to drop some levels.

Simply call factor again on the reduced set

a = factor(c("a","b","b","b"))
a[-1]
factor(a[-1])

If a is in a data frame, replace a by by df$a.

Dieter