Abstract
An observation space S is a family of probability distributions P_i: i∈ I sharing a common sample space Ω in a consistent way. A \emph{grounding} for S is a signed probability distribution P on Ω yielding the correct marginal distribution P_i for every i. A wide variety of quantum scenarios can be formalized as observation spaces. We describe all groundings for a number of quantum observation spaces. Our main technical result is a rigorous proof that Wigner's distribution is the unique signed probability distribution yielding the correct marginal distributions for position and momentum and all their linear combinations.