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. If x does not have a levels attribute then x is returned unchanged regardless of whether or not there were missing values.

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