STUDY OBJECTIVE--The aim was to investigate the pattern of age specific non-response bias in a two phase survey of disablement in the community. It seeks to examine patterns of response in different age groups to a household based postal questionnaire, and the implication of such trends for the estimation of prevalence of reported dependence. It also looks at the effect that the readiness to respond during the first phase postal questionnaire had on participation in the interview based second phase of the study. DESIGN AND SETTING--A two stage survey of disablement in the population was undertaken. A first phase postal questionnaire was sent to 25,168 households in Calderdale, West Yorkshire, England, to ascertain the prevalence of physical disability. The second phase comprised in depth interviews with a sample of individuals identified in the first phase as being disabled. RESPONDENTS--A total of 21,889 postal questionnaires were returned (87%) representing households containing 42,826 people aged 16 years and over. A disproportionately stratified random sample of 950 respondents reporting disability was taken for the second phase. Of these 891 were still available, and 838 (94%) were interviewed. MEASUREMENTS AND MAIN RESULTS--A study of the timing of response to a postal questionnaire showed that patterns differed for different age groups. The estimated prevalence of those aged 65 years and over who were dependent was steady over time whereas for those in the 16-64 age range the estimated prevalence fell as the survey progressed, indicating a tendency for those who were dependent to respond sooner. Examination of the relationship of responses at phase 1 and phase 2 showed that response to invitation to interview was much less in those who had responded later, and presumably more reluctantly, in the first phase. CONCLUSIONS--These findings raise questions about how different patterns of response might be indicative of bias which could differentially affect final age specific prevalence estimates. They also have methodological implications for the follow up of reluctant responders both to increase the response rate and to secure cooperation in the second phase of a two phase survey.
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