Evaluating the evidence for the effectiveness of prevention.

This report reviews a number of issues that should be considered in evaluating the evidence for the effectiveness of primary and secondary prevention. In evaluating primary prevention, changes in disease incidence are the ideal index but mortality may also be a useful surrogate in certain cases. Both relative risk and attributable risk are valuable concepts in evaluating the benefits of primary prevention. In evaluating screening, the most notable approach to secondary prevention, certain potential biases should be taken into account. These include selection biases due to referral or length-biased sampling, lead time bias, and overdiagnosis bias. An understanding of the methodologic issues involved in evaluating prevention will help ensure the validity of any conclusions drawn regarding the effectiveness of preventive measures and programs.