Abstract
An explanation is offered for the reasons why an experienced pathologist engaged in the screening of cervical smears should have issued a large number of false negative diagnoses. The explanation centres on the interactions which occur between successive judgments when a long sequence of similar stimuli are inspected. Briefly, those interactions have the character of an assimilation of each judgment to its predecessor, and the resultant process is capable of “running away”. A procedure is suggested, in which the provision of immediate knowledge of results for a small proportion of the smears inspected would prevent such systematic misdiagnoses from recurring in the future. The implications of this natural experiment for medical screening in general are spelt out.