title: Longterm follow-up in European respiratory health studies - patterns and implications.
abstract: BACKGROUND
  Selection bias is a systematic error in epidemiologic studies that may seriously distort true measures of associations between exposure and disease. Observational studies are highly susceptible to selection bias, and researchers should therefore always examine to what extent selection bias may be present in their material and what characterizes the bias in their material. In the present study we examined long-term participation and consequences of loss to follow-up in the studies Respiratory Health in Northern Europe (RHINE), Italian centers of European Community Respiratory Health Survey (I-ECRHS), and the Italian Study on Asthma in Young Adults (ISAYA).
  
  
  METHODS
  Logistic regression identified predictors for follow-up participation. Baseline prevalence of 9 respiratory symptoms (asthma attack, asthma medication, combined variable with asthma attack and/or asthma medication, wheeze, rhinitis, wheeze with dyspnea, wheeze without cold, waking with chest tightness, waking with dyspnea) and 9 exposure-outcome associations (predictors sex, age and smoking; outcomes wheeze, asthma and rhinitis) were compared between all baseline participants and long-term participants. Bias was measured as ratios of relative frequencies and ratios of odds ratios (ROR).
  
  
  RESULTS
  Follow-up response rates after 10 years were 75% in RHINE, 64% in I-ECRHS and 53% in ISAYA. After 20 years of follow-up, response was 53% in RHINE and 49% in I-ECRHS. Female sex predicted long-term participation (in RHINE OR (95% CI) 1.30(1.22, 1.38); in I-ECRHS 1.29 (1.11, 1.50); and in ISAYA 1.42 (1.25, 1.61)), as did increasing age. Baseline prevalence of respiratory symptoms were lower among long-term participants (relative deviations compared to total baseline population 0-15% (RHINE), 0-48% (I-ECRHS), 3-20% (ISAYA)), except rhinitis which had a slightly higher prevalence. Most exposure-outcome associations did not differ between long-term participants and all baseline participants, except lower OR for rhinitis among ISAYA long-term participating smokers (relative deviation 17% (smokers) and 44% (10-20 pack years)).
  
  
  CONCLUSIONS
  We found comparable patterns of long-term participation and loss to follow-up in RHINE, I-ECRHS and ISAYA. Baseline prevalence estimates for long-term participants were slightly lower than for the total baseline population, while exposure-outcome associations were mainly unchanged by loss to follow-up.
authors.affiliation: Centre for Clinical Research, Haukeland University Hospital, Bergen 5021, Norway. ane.johannessen@helse-bergen.no.
authors.forename: Ane
authors.initials: A
authors.lastname: Johannessen
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Verlato
authors.forename: Bryndis
authors.initials: B
authors.lastname: Benediktsdottir
authors.forename: Bertil
authors.initials: B
authors.lastname: Forsberg
authors.forename: Karl
authors.initials: K
authors.lastname: Franklin
authors.forename: Thorarinn
authors.initials: T
authors.lastname: Gislason
authors.forename: Mathias
authors.initials: M
authors.lastname: Holm
authors.forename: Christer
authors.initials: C
authors.lastname: Janson
authors.forename: Rain
authors.initials: R
authors.lastname: Jögi
authors.forename: Eva
authors.initials: E
authors.lastname: Lindberg
authors.forename: Ferenc
authors.initials: F
authors.lastname: Macsali
authors.forename: Ernst
authors.initials: E
authors.lastname: Omenaas
authors.forename: Francisco Gomez
authors.initials: FG
authors.lastname: Real
authors.forename: Eirunn Waatevik
authors.initials: EW
authors.lastname: Saure
authors.forename: Vivi
authors.initials: V
authors.lastname: Schlünssen
authors.forename: Torben
authors.initials: T
authors.lastname: Sigsgaard
authors.forename: Trude Duelien
authors.initials: TD
authors.lastname: Skorge
authors.forename: Cecilie
authors.initials: C
authors.lastname: Svanes
authors.forename: Kjell
authors.initials: K
authors.lastname: Torén
authors.forename: Marie
authors.initials: M
authors.lastname: Waatevik
authors.forename: Roy Miodini
authors.initials: RM
authors.lastname: Nilsen
authors.forename: Roberto
authors.initials: R
authors.lastname: de Marco
mesh_terms: D000328:Adult; D005060:Europe / Q000453:epidemiology; D005260:Female; D005500:Follow-Up Studies; D006306:Health Surveys; D006801:Humans; D015994:Incidence; D008297:Male; D015995:Prevalence; D011446:Prospective Studies; D012120:Respiration Disorders / Q000453:epidemiology*; D013997:Time Factors; D055815:Young Adult
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