On the basis of their experiences with pregnant patients in their practice, obstetrician/gynecologists estimated the posterior probability of Down syndrome given a positive screening result. They also estimated the base rate of Down syndrome in their practice, along with the hit and false alarm rates for the screening test; for each subject, these numbers were combined to calculate a posterior probability to which the initial estimated posterior probability could be compared. Physicians gave highly consistent estimates when asked to think about their past experiences in terms of event frequencies. However, those told to respond using single event probabilities or to use past experiences to predict prospective frequencies gave inconsistent Bayesian estimates. Thus, when making Bayesian judgments based on real life experience, natural frequency formats only lead to better judgments, compared with single event probability formats, if people think retrospectively, not when using past experiences to make prospective predictions. Copyright © 2011 John Wiley & Sons, Ltd.