  Plausibility measures are structures for reasoning in the face of uncertainty
that generalize probabilities, unifying them with weaker structures like
possibility measures and comparative probability relations. So far, the theory
of plausibility measures has only been developed for classical sample spaces.
In this paper, we generalize the theory to test spaces, so that they can be
applied to general operational theories, and to quantum theory in particular.
Our main results are two theorems on when a plausibility measure agrees with a
probability measure, i.e. when its comparative relations coincide with those of
a probability measure. For strictly finite test spaces we obtain a precise
analogue of the classical result that the Archimedean condition is necessary
and sufficient for agreement between a plausibility and a probability measure.
In the locally finite case, we prove a slightly weaker result that the
Archimedean condition implies almost agreement.
