We prove in a discrete setting that if for all test functions, $t$, there is a $\mathbf{B}$ measurable test function, $s$, such that $E_p(t) = E_p(s)$ for all $p \in P$ then some subfield of $\mathbf{B}$ is sufficient for $P$.
Ann. Statist.
3(5):
1180-1182
(September, 1975).
DOI: 10.1214/aos/1176343249