title: The impact of non-responders on health and lifestyle outcomes in an intervention study.
abstract: BACKGROUND
  A randomized intervention study, "Preventive consultations for 20- to 40-year-old young adults", investigated whether preventive consultations with a general practitioner could help young adults with multiple psychosocial and lifestyle problems to change health behavior. To optimize the response rate of questionnaires at 1 year post-intervention, the non-responders were reminded by telephone. The aim of this study was to examine potential selection bias induced by non-response by comparing responder and non-responder populations at baseline, and to examine the impact on outcomes by comparing initial respondents to respondents after telephone reminding.
  
  
  METHODS
  Non-responders were compared with primary responders using logistic regression models that included socio-demographic factors, health-related factors, and variables related to the intervention study. In order to describe the impact of including responders after telephone reminding on the intervention's effect on different health, resource, and lifestyle outcomes, we compared results in models including and excluding responders after telephone reminding.
  
  
  RESULTS
  Telephone contact raised the response by 10% from 316 (64%) to 364 (74%) among young adults with multiple problems. Being male was the only factor that significantly predicted non-response in the model after adjustment for other variables. The responders after telephone reminding tended to improve health and lifestyle more than the primary responders, but not significantly so. Although the additional responses did not change the estimates of the 1-year effect on health and lifestyle changes, it contributed to increased precision of the results.
  
  
  CONCLUSIONS
  Even though the population of primary non-responders had to some degree a different composition than the primary responders, inclusion of responders after telephone reminding did not significantly change the estimates for effect at the 1-year follow-up; however, the additional responses increased the precision of the estimates.
  
  
  BACKGROUND
  ClinicalTrials.gov: NCT01231256.
authors.affiliation: Department of Social Medicine, Aalborg University Hospital, Havrevangen 1, DK-9000 Aalborg, Denmark. elsebeth.hansen@rn.dk.
authors.forename: Elsebeth
authors.initials: E
authors.lastname: Hansen
authors.forename: Kirsten
authors.initials: K
authors.lastname: Fonager
authors.forename: Kirsten S
authors.initials: KS
authors.lastname: Freund
authors.forename: Jørgen
authors.initials: J
authors.lastname: Lous
mesh_terms: D000328:Adult; D003718:Denmark; D005260:Female; D058006:General Practice; D007722:Health Knowledge, Attitudes, Practice; D006801:Humans; D016015:Logistic Models; D008297:Male; D010349:Patient Compliance; D011314:Preventive Health Services / Q000379:methods*; D012017:Referral and Consultation; D017010:Reminder Systems; D040242:Risk Reduction Behavior; D015983:Selection Bias; D012737:Sex Factors; D011795:Surveys and Questionnaires; D013689:Telephone; D013997:Time Factors; D016896:Treatment Outcome; D035862:Vulnerable Populations / Q000523:psychology*; D055815:Young Adult