  The classic wisdom-of-the-crowd problem asks how a principal can "aggregate"
information about the unknown state of the world from agents without
understanding the information structure among them. We propose a new simple
procedure "\emph{population mean based aggregation}" to achieve this goal. It
only requires eliciting agents' beliefs about the state, and also eliciting
some agents' expectations of the average belief in the population. We show that
this procedure fully aggregates information: in an infinite population, it
always infers the true state of the world. The procedure can accommodate
correlation in agents' information, misspecified beliefs, any finite number of
possible states of the world, and only requires very weak assumptions on the
information structure.
