title: Development of methods to objectively identify time spent using active and motorised modes of travel to work: how do self-reported measures compare?
abstract: BACKGROUND
  Active commuting may make an important contribution to population health. Accurate measures of these behaviours are required, but it is unknown how self-reported estimates compare to those derived from objective measures. We sought to develop methods for objectively deriving time spent in specific travel behaviours from a combination of locational and activity data, and to assess the convergent validity of two self-reported estimates.
  
  
  METHODS
  In 2010 and 2011, a sub-sample of participants from the Commuting and Health in Cambridge study concurrently completed objective monitoring using combined heart rate and movement sensors and global positioning system devices and reported their past-week commuting in a questionnaire (modes used, and usual time spent walking and cycling per trip) and in a day-by-day diary (all modes and durations). Automated and manual approaches were used to objectively identify total time spent using active and motorised modes. Agreement between self-reported and objectively-derived times was assessed using Lin's concordance coefficients, Bland-Altman plots and signed-rank tests.
  
  
  RESULTS
  Compared to objective assessments, day-by-day diary estimates of time spent using active modes on the commute were overestimated by a mean of 1.1 minutes/trip (95% limits of agreement (LOA): -7.7 to 9.9, p < 0.001). The magnitude of overestimation was slightly larger, but not significant (p = 0.247), when walking or cycling was used alone (mean: 2.4 minutes/trip, 95% LOA: -6.8 to 11.5). Total time spent on the commute was overestimated by a mean of 1.9 minutes/trip (95% LOA: -15.3 to 19.0, p < 0.001). The mean differences between self-reported usual time and objective estimates were -1.1 minutes/trip (95% LOA: -8.7 to 6.4) for cycling and +2.4 minutes/trip (95% LOA: -10.9 to 15.7) for walking. Mean differences between usual and daily estimates of time were <1 minute/trip for both walking and cycling.
  
  
  CONCLUSIONS
  We developed a novel method of combining objective data to identify time spent using active and motorised modes, and total time spent commuting. Compared to objectively-derived times, self-reported times spent active commuting were slightly overestimated with wide LOA, suggesting that they should be used with caution to infer aggregate weekly quantities of activity on the commute at the individual level.
authors.affiliation: MRC Epidemiology Unit, School of Clinical Medicine, University of Cambridge, Cambridge Biomedical Campus, Cambridge CB2 0QQ, UK. jenna.panter@mrc-epid.cam.ac.uk.
authors.forename: Jenna
authors.initials: J
authors.lastname: Panter
authors.forename: Silvia
authors.initials: S
authors.lastname: Costa
authors.forename: Alice
authors.initials: A
authors.lastname: Dalton
authors.forename: Andy
authors.initials: A
authors.lastname: Jones
authors.forename: David
authors.initials: D
authors.lastname: Ogilvie
mesh_terms: D000328:Adult; D001642:Bicycling; D004651:Employment; D005260:Female; D040362:Geographic Information Systems; D015438:Health Behavior; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity; D057566:Self Report; D012959:Socioeconomic Factors; D014186:Transportation / Q000379:methods*; D006113:United Kingdom; D016138:Walking