title: Accelerometer data reduction in adolescents: effects on sample retention and bias.
abstract: BACKGROUND
  Accelerometry is increasingly being recognized as an accurate and reliable method to assess free-living physical activity (PA) in children and adolescents. However, accelerometer data reduction criteria remain inconsistent, and the consequences of excluding participants in for example intervention studies are not well described. In this study, we investigated how different data reduction criteria changed the composition of the adolescent population retained in accelerometer data analysis.
  
  
  METHODS
  Accelerometer data (Actigraph GT3X), anthropometric measures and survey data were obtained from 1348 adolescents aged 11-14 years enrolled in the Danish SPACE for physical activity study. Accelerometer data were analysed using different settings for each of the three key data reduction criteria: (1) number of valid days; (2) daily wear time; and (3) non-wear time. The effects of the selected setting on sample retention and PA counts were investigated and compared. Ordinal logistic regression and multilevel mixed-effect linear regression models were used to analyse the impact of differing non-wear time definitions in different subgroups defined by body mass index, age, sex, and self-reported PA and sedentary levels.
  
  
  RESULTS
  Increasing the minimum requirements for daily wear time and the number of valid days and applying shorter non-wear definitions, resulted in fewer adolescents retained in the dataset. Moreover, the different settings for non-wear time significantly influenced which participants would be retained in the accelerometer data analyses. Adolescents with a higher BMI (OR:0.93, CI:0.87-0.98, p=0.015) and older adolescents (OR:0.68, CI:0.49-0.95, p=0.025) were more likely to be excluded from analysis using 10 minutes of non-wear compared to longer non-wear time periods. Overweight and older adolescents accumulated more daily non-wear time if the non-wear time setting was short, and the relative difference between groups changed depending on the non-wear setting. Overweight and older adolescents did also accumulate more sedentary time, but this was not significant correlated to the non-wear setting used.
  
  
  CONCLUSIONS
  Even small differences in accelerometer data reduction criteria can have substantial impact on sample size and PA and sedentary outcomes. This study highlighted the risk of introducing bias with more overweight and older adolescents excluded from the analysis when using short non-wear time definitions.
authors.affiliation: Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark. mtoftager@health.sdu.dk.
authors.forename: Mette
authors.initials: M
authors.lastname: Toftager
authors.forename: Peter Lund
authors.initials: PL
authors.lastname: Kristensen
authors.forename: Melody
authors.initials: M
authors.lastname: Oliver
authors.forename: Scott
authors.initials: S
authors.lastname: Duncan
authors.forename: Lars Breum
authors.initials: LB
authors.lastname: Christiansen
authors.forename: Eleanor
authors.initials: E
authors.lastname: Boyle
authors.forename: Jan Christian
authors.initials: JC
authors.lastname: Brønd
authors.forename: Jens
authors.initials: J
authors.lastname: Troelsen
mesh_terms: D061725:Accelerometry / Q000295:instrumentation* / Q000379:methods*; D000293:Adolescent; D015982:Bias; D015992:Body Mass Index; D001835:Body Weight; D002648:Child; D015331:Cohort Studies; D005260:Female; D006306:Health Surveys; D006801:Humans; D016014:Linear Models; D016015:Logistic Models; D008297:Male; D009043:Motor Activity; D050177:Overweight / Q000628:therapy; D057185:Sedentary Behavior; D057566:Self Report; D013997:Time Factors