Are absolute frequencies, relative frequencies, or both effective in reducing cognitive biases?

Biases in probabilistic reasoning are affected by alterations in the presen tation of judgment tasks. In our experiments, students made likelihood judg ments that an event was produced by various causes. These judgments were ma de in terms of probability, relative frequency or absolute frequency on a f ull or a pruned list of causes. When they had little personal experience of the event (causes of death), the pruning bias was smaller with relative fr equencies than with absolute frequencies or probabilities. When they had mo re personal experience of the event (missing a lecture), the bias was less with both types of frequency than with probability but still lowest with re lative frequency. We suggest that likelihood information is usually stored as relative frequencies when it has been obtained from public sources but t hat it is based on event counts when it is derived from personal experience . Copyright (C) 2000 John Wiley & Sons, Ltd.