title: Non-response in a cross-sectional study of respiratory health in Norway.
abstract: OBJECTIVE
  Declining participation in epidemiological studies has been reported in recent decades and may lead to biased prevalence estimates and selection bias. The aim of the study was to identify possible causes and effects of non-response in a population-based study of respiratory health in Norway.
  
  
  METHODS
  The Telemark study is a longitudinal study that began with a cross-sectional survey in 2013.
  
  
  METHODS
  In 2013, a random sample of 50,000 inhabitants aged 16-50 years, living in Telemark county, received a validated postal questionnaire. The response rate was 33%. In this study, a random sample of 700 non-responders was contacted first by telephone and then by mail.
  
  
  METHODS
  Response rates, prevalence and OR of asthma and respiratory symptoms based on exposure to vapours, gas, dust or fumes (VGDF) and smoking. Causes of non-response.
  
  
  RESULTS
  A total of 260 non-responders (37%) participated. Non-response was associated with younger age, male sex, living in a rural area and past smoking. The prevalence was similar for responders and non-responders for physician-diagnosed asthma and several respiratory symptoms. The prevalence of chronic cough and use of asthma medication was overestimated in the Telemark study, and adjusted prevalence estimates were 17.4% and 5%, respectively. Current smoking was identified as a risk factor for respiratory symptoms among responders and non-responders, while occupational VGDF exposure was a risk factor only among responders. The Breslow-Day test detected heterogeneity between productive cough and occupational VGDF exposure among responders.
  
  
  CONCLUSIONS
  The Telemark study provided valid estimates for physician-diagnosed asthma and several respiratory symptoms, while it was necessary to adjust prevalence estimates for chronic cough and use of asthma medication. Reminder letters had little effect on risk factor associations. Selection bias should be considered in future investigations of the relationship between respiratory outcomes and exposures.
authors.affiliation: Department of Occupational and Environmental Medicine, Telemark Hospital, Skien, Norway Faculty of Medicine, Institute of Clinical Medicine, University of Oslo, Oslo, Norway.
authors.forename: Regine
authors.initials: R
authors.lastname: Abrahamsen
authors.affiliation: Department of Occupational and Environmental Medicine, Telemark Hospital, Skien, Norway.
authors.forename: Martin Veel
authors.initials: MV
authors.lastname: Svendsen
authors.affiliation: National Institute for Occupational Safety and Health (NIOSH), Morgantown, West Virginia, USA.
authors.forename: Paul K
authors.initials: PK
authors.lastname: Henneberger
authors.affiliation: Department of Occupational and Environmental Medicine, Telemark Hospital, Skien, Norway.
authors.forename: Gølin Finckenhagen
authors.initials: GF
authors.lastname: Gundersen
authors.affiliation: Department of Occupational and Environmental Medicine, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden.
authors.forename: Kjell
authors.initials: K
authors.lastname: Torén
authors.affiliation: Faculty of Medicine, Institute of Clinical Medicine, University of Oslo, Oslo, Norway Department of Respiratory Medicine, Oslo University Hospital, Rikshospitalet, Oslo, Norway.
authors.forename: Johny
authors.initials: J
authors.lastname: Kongerud
authors.affiliation: Department of Occupational and Environmental Medicine, Telemark Hospital, Skien, Norway.
authors.forename: Anne Kristin Møller
authors.initials: AK
authors.lastname: Fell
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D001249:Asthma / Q000453:epidemiology; D003371:Cough / Q000453:epidemiology; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D016273:Occupational Exposure / Q000009:adverse effects; D011159:Population Surveillance; D015995:Prevalence; D012140:Respiratory Tract Diseases / Q000453:epidemiology*; D012424:Rural Population; D015983:Selection Bias; D012737:Sex Factors; D012907:Smoking / Q000453:epidemiology; D055815:Young Adult
keywords: RESPIRATORY MEDICINE (see Thoracic Medicine)