Although Berkson's bias is widely recognized in the epidemiologic literature, it remains underappreciated as a model of both selection bias and bias due to missing data. Simple causal diagrams and 2 × 2 tables illustrate how Berkson's bias connects to collider bias and selection bias more generally, and show the strong analogies between Berksonian selection bias and bias due to missing data. In some situations, considerations of whether data are missing at random or missing not at random are less important than the causal structure of the missing data process. Although dealing with missing data always relies on strong assumptions about unobserved variables, the intuitions built with simple examples can provide a better understanding of approaches to missing data in real-world situations.
From the Department of Obstetrics and Gynecology and Duke Global Health Institute, Duke University, Durham, NC.
Submitted 6 May 2011; accepted 9 July 2011; posted 11 November 2011.
Supported by NIH/NICHD 4R00-HD-06-3961 and Duke Center for AIDS Research 2P30-AI-06-4518-06. The author reported no other financial interests related to this research.
Correspondence: Daniel Westreich, Department of Obstetrics and Gynecology, Duke University Medical Center, Box 3084, Durham, NC 27710. E-mail: email@example.com.