Invitation to a population-based cohort study: differences between subjects recruited using various strategies

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Abstract
Aim: The present study compares subjects recruited to a population-based prospective cohort study by community versus personal invitation, and subjects responding late versus early to personal invitation. Subjects were compared with respect to sociodemographic characteristics, selected lifestyle factors, cancer incidence and mortality. Methods: The Malmö Diet and Cancer Study recruited a total of 28,098 subjects between 1991 and 1996. Entire birth-year cohorts, born 1923-50, were recruited using community directed (passive) invitation, and a personal letter of invitation (active recruitment). Information on sociodemography and lifestyle was obtained using a self-administered questionnaire. Anthropometric measurements were assessed at a physical examination. Cancer incidence and cause of death were retrieved using record linkage with national registries. Results: Subjects responding to community directed invitation were older, and more often females, than participants recruited using personal invitation. Furthermore, participants recruited through passive invitation had a comparably more favourable situation with regard to sociodemographic and lifestyle factors. They also had a lower frequency of prevalent disease, lower incidence of cancer and lower mortality. Contrary to this, men were more likely to respond late to a personal letter of invitation and late responders were also older, as compared with subjects responding early to personal invitation. Furthermore, those who responded late had a comparatively unfavourable socioeconomic situation and were characterized by a high prevalence of current smoking, obesity, weight change, and prevalent disease. Conclusions: Subjects who are recruited using different strategies in population-based cohort studies may have different sociodemographic and lifestyle characteristics, and may vary with regard to prevalent disease, cancer incidence and subsequent mortality.