Incomplete ascertainment of all of the eligible patients may also be due to selective recruitment of patients with particular sets of symptoms. In this example, practices with lower rates of recruitment tended to recruit patients with more severe symptoms. The reasons for this apparent selection bias are unknown,although possible explanations include that (1) there was variability in the definition of the disease; (2) variability in the interpretation of the eligibility criteria; or (3) practices that had suspended recruitment were reminded about the study when a patient with severe disease presented.