  The assumption that we are typical observers plays a core role in attempts to
make multiverse theories empirically testable. A widely shared worry about this
assumption is that it suffers from systematic ambiguity concerning the
reference class of observers with respect to which typicality is assumed. As a
way out, Srednicki and Hartle recommend that we empirically test typicality
with respect to different candidate reference classes in analogy to how we test
physical theories. Unfortunately, as this paper argues, this idea fails because
typicality is not the kind of assumption that can be subjected to empirical
tests. As an alternative, a background information constraint on observer
reference class choice is suggested according to which the observer reference
class should be chosen such that it includes precisely those observers who one
could possibly be, given one's assumed background information.
