[AccessD] (Yes/No) field - no state A97

I've seen some discussion on this throughout the ages but I've never paid
attention to it since it would only seem to be needed when a design is
flawed.

But...

I have a project which is almost complete that uses a number of Yes/No
fields and the client just informed me that there is a condition where
neither would be correct.

The form is part of an evaluation of conditions. The conditions can only be
yes or no but, in the endless glory of government programs, if a partial
evaluation had been done before the latest standards the evaluation can be
completed with a "Not Evaluated" given to certain areas.

I thought maybe I could just have the final report evaluate nulls in the
Yes/No field as "Not Evaluated" areas. The "Triple State" isn't making much
sense to me. Can anyone clear it up for me or should I just change my last
minute change approach and add a new field?

John B.
PS: There isn't any additional money to be had so either I do it easily or I
eat the expense. Even though it was explained to me that it could only be
yes or no, it isn't explicitly stated anywhere in the specs and the contact
person is gone now, so...