title: How many days of pedometer use predict the annual activity of the elderly reliably?
abstract: OBJECTIVE
  Daily variations of physical activity in the elderly remain unclear. We thus used a uniaxial accelerometer/pedometer to examine the variability of step counts for 1 yr, determining the minimum number of days observation needed to obtain reliable estimates of annual physical activity.
  
  
  METHODS
  Subjects were 37 males and 44 females, healthy Japanese, aged 65-83 yr. The pedometer was worn on the waistband throughout 1 yr, accumulating information on the individual's daily step count.
  
  
  RESULTS
  The step count spectrum showed peaks with periods of 2.3, 3.5, and 7.0 d and an aperiodic component that had a greater power at low frequencies (i.e., non-white noise). These characteristics were absent in randomly resequenced data. To ensure that 80% of total variance was attributable to between-subjects variance, 25 and 8 consecutive days of observation were needed in male and female subjects, respectively. To achieve 90% on this same measure of reliability, 105 and 37 consecutive days of observation were required. In contrast, 4 d of randomly timed observations yielded 80% reliability for both men and women, and 11 and 9 d gave 90% reliability in men and women, respectively. If sampling also took account of season and day of the week, the respective observation periods for men and women were reduced to 8 and 4 d (i.e., 2 and 1 consecutive days of sampling every 89 d) for 80% and to 16 and 12 d (i.e., 4 and 3 consecutive days every 89 d) for 90% reliability.
  
  
  CONCLUSIONS
  When estimating annual step counts, seasonal and/or random sampling of data allows collection of reliable data during substantially fewer days than needed for consecutive observations.
authors.affiliation: Department of Work Stress Control, Japan National Institute of Occupational Safety and Health, Kanagawa, Japan.
authors.forename: Fumiharu
authors.initials: F
authors.lastname: Togo
authors.forename: Eiji
authors.initials: E
authors.lastname: Watanabe
authors.forename: Hyuntae
authors.initials: H
authors.lastname: Park
authors.forename: Akitomo
authors.initials: A
authors.lastname: Yasunaga
authors.forename: Sungjin
authors.initials: S
authors.lastname: Park
authors.forename: Roy J
authors.initials: RJ
authors.lastname: Shephard
authors.forename: Yukitoshi
authors.initials: Y
authors.lastname: Aoyagi
mesh_terms: D000368:Aged / Q000502:physiology*; D005260:Female; D005583:Fourier Analysis; D006801:Humans; D007899:Leisure Activities; D008137:Longitudinal Studies; D008297:Male; D018670:Monitoring, Ambulatory / Q000379:methods*; D009043:Motor Activity; D011237:Predictive Value of Tests; D012621:Seasons; D016138:Walking