Replace NA's in a Factor with a New Level
DESCRIPTION:
Creates an additional level in a new object representing the missing values in the old object.
USAGE: 
na.include(x)
REQUIRED ARGUMENTS:
x:
a factor or any object inheriting from "factor".
VALUE:
an object like x, but with an additional level, "NA", representing all the missing values in x. If there were no missing values, x is returned.
EXAMPLES: 
attach(market.frame)
table(reach.out)
   N  Y 2
 919 62 2


table(na.include(reach.out))
   N  Y 2 NA
 919 62 2 17