Probability and conclusions

[Dancingbambarina]

What are the rules in concluding from a contrapositive pertaining to probability. I see examples where it's okay, and examples where it's not (where groups or numbers are usually involved) to conclude from the contrapositive the probability matching in the conclusion from the normal form. So just to be clear, the negated necessary condition goes to the sufficient but the probabiity stays in the negated sufficient - now the necessary. So one term in a sense "hogs" the probability factor.

Thank you