According to the TEE model for grades of membership
(see Fuzzy Sets Systems vol 25, pp. 325-348, March 1988)
the possibility or grade of membership
mu (B) (where, e.g. A=tall = element of a complete and
A nonredundant set of labels)
and B=180cm=element of a height universe)
should be interpreted roughly as P(A|B)=probability of assignment of
the label A to a person of height B.
>From the theory of probability we then have
P(A,B)=P(A)P(B|A)=P(B)(A|B)
where P(A)=sum of P(A,B) over all elements of the B universe,
where P(B)=sum of P(A,B) over all elements of the A universe.
Your two universes consist of two elements each, {A+, A-}
{B+, B-}.
The above formulas are then valid for A=either A+ or A-
and similarly for B.
Greetings,
Ellen Hisdal