title: Reproducibility of Accelerometer-Assessed Physical Activity and Sedentary Time.
abstract: BACKGROUND
  Accelerometers are used increasingly in large epidemiologic studies, but, given logistic and cost constraints, most studies are restricted to a single, 7-day accelerometer monitoring period. It is unknown how well a 7-day accelerometer monitoring period estimates longer-term patterns of behavior, which is critical for interpreting, and potentially improving, disease risk estimates in etiologic studies.
  
  
  METHODS
  A subset of participants from the Women's Health Study (N=209; mean age, 70.6 [SD=5.7] years) completed at least two 7-day accelerometer administrations (ActiGraph GT3X+) within a period of 2-3 years. Monitor output was translated into total counts, steps, and time spent in sedentary, light-intensity, and moderate to vigorous-intensity activity (MVPA) and bouted-MVPA (i.e., 10-minute bouts). For each metric, intraclass correlations (ICCs) and 95% CIs were calculated using linear-mixed models and adjusted for wear time, age, BMI, and season. The data were collected in 2011-2015 and analyzed in 2015-2016.
  
  
  RESULTS
  The ICCs ranged from 0.67 (95% CI=0.60, 0.73) for bouted-MVPA to 0.82 (95% CI=0.77, 0.85) for total daily counts and were similar across age, BMI, and for less and more active women. For all metrics, classification accuracy within 1 quartile was >90%.
  
  
  CONCLUSIONS
  These data provide reassurance that a 7-day accelerometer-assessment protocol provides a reproducible (and practical) measure of physical activity and sedentary time. However, ICCs varied by metric; therefore, future prospective studies of chronic diseases might benefit from existing methods to adjust risk estimates for within-person variability in activity to get a better estimate of the true strength of association.
authors.affiliation: Metabolic Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, Maryland; Cancer Prevention Fellowship Program, Division of Cancer Prevention, National Cancer Institute, Bethesda, Maryland. Electronic address: skeadle@calpoly.edu.
authors.forename: Sarah Kozey
authors.initials: SK
authors.lastname: Keadle
authors.affiliation: Laboratory of Epidemiology and Population Sciences, Intramural Research Program, National Institute of Aging, Bethesda, Maryland.
authors.forename: Eric J
authors.initials: EJ
authors.lastname: Shiroma
authors.affiliation: Division of Preventive Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts; National Institute of Health and Nutrition, Tokyo, Japan.
authors.forename: Masamitsu
authors.initials: M
authors.lastname: Kamada
authors.affiliation: Metabolic Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, Maryland.
authors.forename: Charles E
authors.initials: CE
authors.lastname: Matthews
authors.affiliation: Laboratory of Epidemiology and Population Sciences, Intramural Research Program, National Institute of Aging, Bethesda, Maryland.
authors.forename: Tamara B
authors.initials: TB
authors.lastname: Harris
authors.affiliation: Division of Preventive Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts; Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, Massachusetts.
authors.forename: I-Min
authors.initials: IM
authors.lastname: Lee
mesh_terms: D061725:Accelerometry; D000368:Aged; D015444:Exercise; D005260:Female; D006801:Humans; D015203:Reproducibility of Results; D057185:Sedentary Behavior