Abstract
How much risk can we expose our research subjects to? There is a special challenge answering this question when the evidence on which we base our assessments of risk is fragmentary, conflicting or sparse. Such evidence does not support precise assignments of risk (eg, there is a 24.8% chance that this patient will develop AIDS in the next year if she participates in my study). At best it supports imprecise assignments of risk (eg, there is between a 5% and 35% chance that this patient will develop AIDS in the next year if she participates in my study). Here I discuss three approaches to evaluating risk when probability assignments are imprecise-an optimistic approach, a moderate approach and a pessimistic approach. I offer a practical reason to favour the pessimistic approach.