Order Unit Spaces and Probabilistic Models

We exhibit a functor from the category OUS of order unit spaces and positive, unit-preserving mappings into the category $\text{Prob}$ of probabilistic models (test spaces with designated state spaces) and morphisms thereof. Restricted to any subcategory of OUS monoidal with respect to a positive, normalized, bilinear composition rule, our functor is also monoidal. This shows that the convex-operational approach to physical theories can be subsumed by the test-space approach, without resort to generalized test spaces''. A second construction, equipping a probabilistic model with tests representing weighted coins’', also sheds light on the nature of unsharp observables.