The mechanism design literature assumes too much common knowledge of the en- vironment among the players and planner.We relax this assumption by studying mech- anism design on richer type spaces. We ask when ex post implementation is equivalent to interim (or Bayesian) imple- mentation for all possible type spaces. The equivalence holds in the case of separable environments; examples of separable environments arise (1) when the planner is im- plementing a social choice function (not correspondence) and (2) in a quasilinear en- vironment with no restrictions on transfers. The equivalence fails in general, including in some quasilinear environments with budget balance. In private value environments, ex post implementation is equivalent to dominant strategies implementation. The private value versions of our results offer new insights into the relationship between dominant strategy implementation and Bayesian imple- mentation.
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