An empirical demonstration of Berkson's bias

The importance of Berkson's bias (a systematic bias in the distribution of disease amongst hospitalized patients) in certain types of hospital based research has not been substantiated with empirical data. This paper firstly reviews the theoretical origins of the bias and secondly attempts to provide supportive empirical evidence. The basis of the analysis has been data generated in household surveys designed to capture health utilization information. Sufficient data were available for eight broad clinical conditions and six medications. All possible pairs of associations have been examined in both the entire data set and the hospitalized subgroups; statistically significant differences in relative risk were then identified. Berkson's theory provides a relatively weak yet statistically significant predictive validity for the change in relative risk from community to hospital.