Validity of claims to lifelong non-smoking at age 36 in a longitudinal study.

Consistency of self-reported cigarette smoking habits was investigated in the MRC National Survey of Health and Development, using data collected at ages 20, 25, 31 and 36 years. Of those claiming to be lifelong non-smokers at age 36, 7.9% had previously reported smoking at least one cigarette a day. Those cohort members with inconsistent records had smoked fewer cigarettes and were less likely to inhale than cohort members who smoked at age 36, and had started smoking later. Re-analysis of data collected by Friedman et al suggested that the effect of misclassification of ex-smokers was to reduce the relative risk of death from all causes, for men and women aged 35-54 years, by a factor of 5%. It is suggested that in future studies, those claiming to be lifelong non-smokers should be questioned about their habits in late adolescence and early adulthood.