title: Is measurement error altered by participation in a physical activity intervention?
abstract: OBJECTIVE
  There is no "gold standard" measure for moderate to vigorous physical activity (MVPA); some error is inherent to self-report and device-based measures. Few studies have examined agreement between self-report and device-based measures in the intervention trial context or whether the difference between measures is influenced by intervention participation.
  
  
  METHODS
  MVPA was measured at baseline and after 6 months by Active Australia Survey (AAS) and by the GT1M accelerometer (≥1952 counts per minute) in the intervention (n = 135) and usual care control (n = 141) participants of a randomized trial targeting weight loss by MVPA increases and energy intake reductions in adults with type 2 diabetes. Agreement (for each group at each assessment) was examined using the Bland-Altman approach and regression-based modeling. Because the differences between MVPA measures varied with average values ([AAS + GT1M] / 2), they were examined as a percentage of average physical activity. t-tests were used to assess unadjusted group differences and changes over time. ANCOVA models tested intervention effects on measurement error at follow-up, adjusted for baseline.
  
  
  RESULTS
  Agreement worsened, and variability in the difference measures became greater, as the average amount of MVPA increased. Measurement error differed significantly between groups at follow-up (P = 0.010) but not at baseline (P = 0.157) and changed significantly within the intervention group (P = 0.001) but not the control group (P = 0.164). There was a statistically significant effect of the intervention on measurement error (P = 0.026).
  
  
  CONCLUSIONS
  Measurement error of self-report relative to the accelerometer appeared to be affected by intervention. Because measurement error cannot be definitively attributed to self-report or accelerometer, it would be prudent to measure both in future studies.
authors.affiliation: The University of Queensland, School of Population Health, Cancer Prevention Research Centre, Brisbane, Australia.
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Winkler
authors.forename: Lauren
authors.initials: L
authors.lastname: Waters
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Eakin
authors.forename: Brianna
authors.initials: B
authors.lastname: Fjeldsoe
authors.forename: Neville
authors.initials: N
authors.lastname: Owen
authors.forename: Marina
authors.initials: M
authors.lastname: Reeves
mesh_terms: D061725:Accelerometry; D000368:Aged; D003376:Counseling; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity; D015203:Reproducibility of Results; D057566:Self Report