Re: Removing levels from a factor

[Terry Therneau <therneau@mayo.edu>]

 The easiest thing is to turn off factors:
 
 > options(stringsAsFactors=F)
 > data$Treatment <- as.character(data$Treatment)
 
 Now you can subset the data frame and things will work as you would anticipate.
 
 Comment: factors are occassionaly useful, but only occasionally.  Much grief 
can be avoided by turning them off by default.  Our biostat group (>100 people, 
over 1200 projects a year) has had the above option as a part of our global 
defaults for many years, and has not yet seen a downside to the decision.
 
        Terry Therneau
        Mayo Clinic