best output representation?

[Scott_Fahlman@SEF-PMAX.SLISP.CS.CMU.EDU]

    Hi all.  Let's say you have a classification problem that can
    be answered with one of three mutually exclusive alternatives
    (eg, yes, no, don't care)
    
    Of the following output representations:
    ...
    which is best, and for what reason?
    
I would say that the correct answer is "none of the above".  What you want
to do is use a single output, with +1 meaning "yes" and -1 meaning "no".
Take all the "don't care" cases out of the training set, because you don't
care what answer they give you.  (And you may as well take them out of the
test set as well, since they will always be correct.)

This will make training faster because (a) the training set is smaller and
(b) it allows the net to concentrate all its resources on getting the "do
care" cases right.

-- Scott Fahlman