Effects of a sequential mixed-mode design on participation, contact and sample composition: Results of the pilot study "IMOA - Improving Health Monitoring in Old Age"

Abstract

Existing health survey data of individuals who are 65+ years of age is limited due to the exclusion of the oldest old and physically or cognitively impaired individuals. This study aimed to assess the effects of a sequential mixed-mode design on (1) contact and response rates, (2) sample composition and (3) non-response bias. A register-based random sample of 2,000 individuals 65+ years was initially contacted by mail to answer a health questionnaire. Random subgroups of initial non-responders were further contacted by telephone or home visits. Participation by interview or proxy was possible. After postal contact only, the initial contact and response rates were 51.7% and 37.8%, respectively. The contact and response rates increased to 71.1% and 44.6%, respectively, after all contact steps. A different sample composition regarding sociodemographic (i.e., older individuals) and health characteristics (i.e., worse self-rated health, more functional impairments) was achieved by the inclusion of those late participants. Ill health was the second most frequent reason for nonparticipation. Personal contact modes are important to increase contact and response rates in population-based health studies and to include hard-to-reach groups such as the oldest or physically impaired individuals. However, non-response bias still occurred.