Non-responders to a postal questionnaire on respiratory symptoms and diseases

Only few data have been published about non-responders in epidemiological studies on respiratory diseases. The aim of this study was to examine the characteristics of the non-responders and the reasons for non-response in a survey of respiratory health. All 9132 subjects, born 1925-1926, 1940-1941, 1955-1956 and 1970-1971 and living in eight representative areas in Norrbotten, Sweden, were invited to a prevalence study on asthma, chronic bronchitis and respiratory symptoms. The response rate was 85%. A sample of the non-responders from the cross-sectional postal questionnaire study was contacted by telephone and interviewed using the same questionnaire as had been used in the postal survey. Of the 1397 non-responders a stratified sample of 182 subjects were selected for this study and 144 agreed to participate. The response rate was increasing by increasing age. The main reason for non-response was that the subjects had forgotten to mail the questionnaire, lack of interest or lack of time. There were significantly higher proportions of current smokers and manual workers among the non-responders. The prevalence rates of wheezing, longstanding cough, sputum production, attacks of breathlessness, asthma and use of asthma medicines were significantly higher among the non-responders compared with the responders according both univariate and multivariate logistic regression analyses, in which the influences of age, sex, smoking habits, socioeconomic group and area of domicile were taken into account. The prevalence of respiratory symptoms and diseases was slightly underestimated in the postal survey.