The major strengths of the present study are the enrollment of incident cases and randomly sampled population controls; the large sample size; the unusually high participation rates, especially considering that biologic samples were requested; and the face-to-face collection of detailed information with a structured questionnaire by trained interviewers. Still, in interpreting the results, we considered several possible sources of bias. Reliability of self reported job history is usually considered good (12, 15) and not a source of important recall bias. Blind coding of occupations eliminated the possibility of differential bias,although a certain degree of nondifferential misclassification is practically unavoidable, leading to an average bias toward the null (11, 49, 50). We exploited the detailedinterview data to adjust for different smoking-related characteristics.
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