 it would be most unrealistic to suppose that we can fulfill the requirement of selecting the broadest homogeneous reference class in all cases in which we have to make practical decisions about single events. We may suspect that a given reference class is inhomogeneous, but not know of any way to make a relevant partition of it. Under these circumstances let us say that the class is epistemically homogeneous. […] Sometimes we know that a reference class is inhomogeneous, but it would simply be impractical to carry out a relevant subdivision. […] Under these circumstances, let us say the reference class is practically homogeneous. (Salmon 1967a: 92)