Berkson's bias revisited.

When used in a loose manner to indicate distortive effects on associations among hospitalized patients, the term Berkson's bias denotes a special case of Simpson's paradox. If, however, Berkson's independence assumption is introduced, Berkson's bias affects only the "selected" subjects and not those "left behind" and tends to decrease the odds-ratio. Generally speaking, the model is valid not only for case-control studies but also for prospective and other investigations. By introducing a time dimension the model allows for the study of changes over time in Berkson's bias.