title: The effect of a physical activity intervention on bias in self-reported activity.
abstract: OBJECTIVE
  A positive outcome in self-reported behavior could be detected erroneously if an intervention caused over-reporting of the targeted behavior. Data collected from a multi-site randomized trial were examined to determine if adolescent girls who received a physical activity intervention over-reported their activity more than girls who received no intervention.
  
  
  METHODS
  Activity was measured using accelerometers and self-reports (3-Day Physical Activity Recall, 3DPAR) in cross-sectional samples preintervention (6th grade, n = 1,464) and post-intervention (8th grade, n = 3,114). Log-transformed accelerometer minutes were regressed on 3DPAR blocks, treatment group, and their interaction, while adjusting for race, body mass index, and timing of data collection.
  
  
  RESULTS
  Preintervention, the association between measures did not differ between groups, but post-intervention 3DPAR blocks were associated with fewer log-accelerometer minutes of moderate-vigorous physical activity (MVPA) in intervention girls than in control girls (p = 0.002). The group difference was primarily in the upper 15% of the 3DPAR distribution, where control girls had >1.7 more accelerometer minutes of MVPA than intervention girls who reported identical activity levels. Group differences in this subsample were 8.5%-16.2% of the mean activity levels; the intervention was powered to detect a difference of 10%.
  
  
  CONCLUSIONS
  Self-report measures should be interpreted with caution when used to evaluate a physical activity intervention.
authors.affiliation: Department of Epidemiology, University of North Carolina, Chapel Hill, NC 27599, USA. dtaber@email.unc.edu
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Taber
authors.forename: June
authors.initials: J
authors.lastname: Stevens
authors.forename: David M
authors.initials: DM
authors.lastname: Murray
authors.forename: John P
authors.initials: JP
authors.lastname: Elder
authors.forename: Larry S
authors.initials: LS
authors.lastname: Webber
authors.forename: Jared B
authors.initials: JB
authors.lastname: Jobe
authors.forename: Leslie A
authors.initials: LA
authors.lastname: Lytle
mesh_terms: D000293:Adolescent; D000294:Adolescent Behavior / Q000523:psychology*; D015982:Bias; D002648:Child; D003430:Cross-Sectional Studies; D003627:Data Interpretation, Statistical; D005260:Female; D006801:Humans; D011939:Mental Recall; D018670:Monitoring, Ambulatory / Q000295:instrumentation / Q000379:methods; D009043:Motor Activity; D012044:Regression Analysis; D012650:Self Disclosure; D012928:Social Desirability