In the Northern Ireland PRIME centre—Belfast—the sampling frame was based on industry, the civil service, and general practice. In the three French centres—Lille, Strasbourg, and Toulouse—the sampling frame was health screening centres for free medical examinations (organised by the social security system and offered to each person) and occupational medicine settings where each worker was examined annually. Each sample was recruited to match broadly the social class structure of the background population. Every man selected was invited to participate in the study, and participation was voluntary. Individuals were informed of the aim of the study and signed a consent form that included agreement to annual follow-up. Approval from the appropriate local ethics committee was obtained in each study centre.25
