title: Non-response bias in physical activity trend estimates.
abstract: BACKGROUND
  Increases in reported leisure time physical activity (PA) and obesity have been observed in several countries. One hypothesis for these apparently contradictory trends is differential bias in estimates over time. The purpose of this short report is to examine the potential impact of changes in response rates over time on the prevalence of adequate PA in Canadian adults.
  
  
  METHODS
  Participants were recruited in representative national telephone surveys of PA from 1995-2007. Differences in PA prevalence estimates between participants and those hard to reach were assessed using Student's t tests adjusted for multiple comparisons.
  
  
  RESULTS
  The number of telephone calls required to reach and speak with someone in the household increased over time, as did the percentage of selected participants who initially refused during the first interview attempt. A higher prevalence of adequate PA was observed with 5-9 attempts to reach anyone in the household in 1999-2002, but this was not significant after adjustment for multiple comparisons.
  
  
  CONCLUSIONS
  No significant impact on PA trend estimates was observed due to differential non response rates. It is important for health policy makers to understand potential biases and how these may affect secular trends in all aspects of the energy balance equation.
authors.affiliation: Canadian Fitness and Lifestyle Research Institute, Ottawa, Canada. ccraig@cflri.ca
authors.forename: Cora L
authors.initials: CL
authors.lastname: Craig
authors.forename: Christine
authors.initials: C
authors.lastname: Cameron
authors.forename: Joe
authors.initials: J
authors.lastname: Griffiths
authors.forename: Adrian
authors.initials: A
authors.lastname: Bauman
authors.forename: Catrine
authors.initials: C
authors.lastname: Tudor-Locke
authors.forename: Ross E
authors.initials: RE
authors.lastname: Andersen
mesh_terms: D000328:Adult; D015982:Bias; D002170:Canada; D015444:Exercise; D005260:Female; D006306:Health Surveys; D006801:Humans; D008297:Male; D011795:Surveys and Questionnaires; D013689:Telephone