Economists studying choice with partial knowledge typically assume that the decision maker places

a subjective distribution on unknown quantities and maximizes expected utility.  Someone lacking

a subjective distribution faces a problem of choice under ambiguity.  This article reviews recent

research on policy choice under ambiguity, when the task is to choose treatments for a population.

Ambiguity arises when a planner has partial knowledge of treatment response and does not feel able

to place a subjective distribution on the unknowns.  I first discuss dominance and alternative criteria

for choice among undominated policies.  I then illustrate with choice of a vaccination policy by a

planner who has partial knowledge of the effect of vaccination on illness.  I next study a class of

problems where a planner may want to cope with ambiguity by diversification, assigning

observationally identical persons to different treatments.  Lastly, I consider a setting where a planner

should not diversify treatment.