title: Reliability of accelerometer-determined physical activity and sedentary behavior in school-aged children: a 12-country study.
abstract: OBJECTIVE
  Focused on the accelerometer-determined physical activity and sedentary time metrics in 9-11-year-old children, we sought to determine the following: (i) number of days that are necessary to achieve reliable estimates (G⩾0.8); (ii) proportion of variance attributed to different facets (participants and days) of reliability estimates; and (iii) actual reliability of data as collected in The International Study of Childhood Obesity, Lifestyle and Environment (ISCOLE).
  
  
  METHODS
  The analytical sample consisted of 6025 children (55% girls) from sites in 12 countries. Physical activity and sedentary time metrics measures were assessed for up to 7 consecutive days for 24 h per day with a waist-worn ActiGraph GT3X+. Generalizability theory using R software was used to investigate the objectives i and ii. Intra-class correlation coefficients (ICC) were computed using SAS PROC GLM to inform objective iii.
  
  
  RESULTS
  The estimated minimum number of days required to achieve a reliability estimate of G⩾0.8 ranged from 5 to 9 for boys and 3 to 11 for girls for light physical activity (LPA); 5 to 9 and 3 to 10, for moderate-to-vigorous physical activity (MVPA); 5 to 10 and 4 to 10 for total activity counts; and 7 to 11 and 6 to 11 for sedentary time, respectively. For all variables investigated, the 'participant' facet accounted for 30-50% of the variability, whereas the 'days' facet accounted for ⩽5%, and the interaction (P × D) accounted for 50-70% of the variability. The actual reliability for boys in ISCOLE ranged from ICCs of 0.78 to 0.86, 0.73 to 0.85 and 0.72 to 0.86 for LPA, MVPA and total activity counts, respectively, and 0.67 to 0.79 for sedentary time. The corresponding values for girls were 0.80-0.88, 0.70-0.89, 0.74-0.86 and 0.64-0.80.
  
  
  CONCLUSIONS
  It was rare that only 4 days from all participants would be enough to achieve desirable reliability estimates. However, asking participants to wear the device for 7 days and requiring ⩾4 days of data to include the participant in the analysis might be an appropriate approach to achieve reliable estimates for most accelerometer-derived metrics.
authors.affiliation: Department of Exercise Science, School of Education, University of Syracuse, Syracuse, NY, USA; Pennington Biomedical Research Center, Baton Rouge, LA, USA.
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authors.affiliation: Pennington Biomedical Research Center, Baton Rouge, LA, USA; School of Biological and Population Health Sciences Oregon State University, Corvallis, OR, USA.
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authors.affiliation: Pennington Biomedical Research Center, Baton Rouge, LA, USA; Department of Kinesiology, University of Massachusetts Amherst, Amherst, MA, USA.
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authors.affiliation: Alliance for Research in Exercise Nutrition and Activity (ARENA), School of Health Sciences, University of South Australia , Adelaide, Australia.
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authors.affiliation: CIFI2D, Faculdade de Desporto, University of Porto , Porto, Portugal.
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authors.affiliation: Centro de Estudos do Laboratório de Aptidão Física de São Caetano do Sul (CELAFISCS) , Sao Paulo, Brazil.
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authors.affiliation: Alliance for Research in Exercise Nutrition and Activity (ARENA), School of Health Sciences, University of South Australia , Adelaide, Australia.
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authors.affiliation: Department of Recreation Management and Exercise Science, Kenyatta University , Nairobi, Kenya.
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authors.affiliation: School of Medicine, Universidad de los Andes , Bogota, Colombia.
authors.forename: O L
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authors.affiliation: Department for Health, University of Bath , Bath, UK.
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authors.affiliation: Children's Hospital of Eastern Ontario Research Institute , Ottawa, Ontario, Canada.
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authors.affiliation: Pennington Biomedical Research Center , Baton Rouge, LA, USA.
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