PHYSICIANS frequently must provide patients with estimates of the risk of a particular medical procedure or of the probability of a specific health outcome. To do so, they may use data from their own experiences as well as those available in the literature. A general goal is to locate the estimate within a fairly narrow range.
When the procedure or outcome of concern is relatively uncommon, precise estimation, although desirable, may be difficult or impossible. Moreover, when the only data available come from a study in which none of the events of concern actually occurred—ie, a study in which a zero numerator is reported—making inferences seems to be particularly problematic. Because the occurrence of "no events" seems to be viewed as very different both quantitatively and qualitatively from the occurrence of one or more events, it is useful to look into someof the statistical and psychological issues that influence the