Baseline characteristics are not sufficient indicators of non-response bias follow up studies.

STUDY OBJECTIVE The aim was to examine whether baseline characteristics from a cross sectional survey provided sufficient information regarding non-response bias in a follow up study when compared with information on hospital admissions in the intervening years. DESIGN This was an 11 year follow up study of a cohort selected in 1974 with register information on hospital admissions during follow up. SETTING The study was based on a sample of cement workers from a particular Portland cement factory with suitable controls from other occupations. PARTICIPANTS A total of 1404 men participated in the first survey in 1974, including a questionnaire and lung function tests. In 1985 1070 men were alive and of these, 928 men (87%) responded to a postal questionnaire. MAIN RESULTS Non-responders in 1985 did not differ markedly from responders when smoking habits, respiratory symptoms, and lung function were examined in 1974. During follow up, non-responders had twice as high rates of hospital admission due to respiratory diseases as responders. These differences remained present after adjusting for minor differences in age and smoking habits. CONCLUSIONS Equal distributions of baseline characteristics among responders and non-responders in a follow up study do not preclude non-response bias.