Converts a factor to a numeric vector with approximately the
numeric values of its levels. Hence, the levels of the
factor must be numeric values, stored as characters. It uses the method
described in factor. Use as.numeric to convert a
factor to a numeric vector with integers 1, 2, ... corresponding
to the positions in the list of levels. You can also use fac.recode to
recode the levels to numeric values. If a numeric is supplied, it is
left unchanged.