title: Investigation of relative risk estimates from studies of the same population with contrasting response rates and designs.
abstract: BACKGROUND
  There is little empirical evidence regarding the generalisability of relative risk estimates from studies which have relatively low response rates or are of limited representativeness. The aim of this study was to investigate variation in exposure-outcome relationships in studies of the same population with different response rates and designs by comparing estimates from the 45 and Up Study, a population-based cohort study (self-administered postal questionnaire, response rate 18%), and the New South Wales Population Health Survey (PHS) (computer-assisted telephone interview, response rate ~60%).
  
  
  METHODS
  Logistic regression analysis of questionnaire data from 45 and Up Study participants (n = 101,812) and 2006/2007 PHS participants (n = 14,796) was used to calculate prevalence estimates and odds ratios (ORs) for comparable variables, adjusting for age, sex and remoteness. ORs were compared using Wald tests modelling each study separately, with and without sampling weights.
  
  
  RESULTS
  Prevalence of some outcomes (smoking, private health insurance, diabetes, hypertension, asthma) varied between the two studies. For highly comparable questionnaire items, exposure-outcome relationship patterns were almost identical between the studies and ORs for eight of the ten relationships examined did not differ significantly. For questionnaire items that were only moderately comparable, the nature of the observed relationships did not differ materially between the two studies, although many ORs differed significantly.
  
  
  CONCLUSIONS
  These findings show that for a broad range of risk factors, two studies of the same population with varying response rate, sampling frame and mode of questionnaire administration yielded consistent estimates of exposure-outcome relationships. However, ORs varied between the studies where they did not use identical questionnaire items.
authors.affiliation: The Sax Institute, Broadway NSW, Australia. Nicole.Mealing@saxinstitute.org.au
authors.forename: Nicole M
authors.initials: NM
authors.lastname: Mealing
authors.forename: Emily
authors.initials: E
authors.lastname: Banks
authors.forename: Louisa R
authors.initials: LR
authors.lastname: Jorm
authors.forename: David G
authors.initials: DG
authors.lastname: Steel
authors.forename: Mark S
authors.initials: MS
authors.lastname: Clements
authors.forename: Kris D
authors.initials: KD
authors.lastname: Rogers
mesh_terms: D000368:Aged; D005260:Female; D005794:Generalization, Response; D006302:Health Services Research; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D009517:New South Wales; D016017:Odds Ratio; D044382:Population Groups; D012107:Research Design; D012306:Risk; D012307:Risk Factors; D011795:Surveys and Questionnaires