Berkson fallacy revisited. Spurious conclusions from patient surveys.

Clinical surveys of "selected" convenient patient samples (hospital or clinic) can lead to spurious conclusions unless the relationship of selected patients to the source population is carefully controlled. The possibility of biased conclusions from uneven selection rates of patients is called Berkson fallacy. Studies of cerebral palsy and low birth weight provide clear examples of Berkson fallacy. A fictitious clinical survey of cerebral palsy and low birth weight is presented to illustrate how Berkson fallacy may lead to interesting but spurious conclusions from patient surveys.