  We quantify the intrinsic noise content of an observable in a general
probabilistic theory and derive a noise content inequality for incompatible
observables. We apply the derived inequality to standard quantum theory, the
quantum theory of processes, and polytope state spaces. The noise content for
positive operator-valued measures takes a particularly simple form and equals
the sum of minimal eigenvalues of all the effects. We illustrate our findings
with a number of examples including the introduced notion of reverse
observables.
