title: Accelerometer test-retest reliability by data processing algorithms: results from the Twin Cities Walking Study.
abstract: BACKGROUND
  The purpose of this study was to determine 1) the test-retest reliability of adult accelerometer-measured physical activity, and 2) how data processing decisions affect physical activity levels and test-retest reliability.
  
  
  METHODS
  143 people wore the ActiGraph accelerometer for 2 7-day periods, 1 to 4 weeks apart. Five algorithms, varying nonwear criteria (20 vs. 60 min of 0 counts) and minimum wear requirements (6 vs. 10 hrs/day for ≥ 4 days) and a separate algorithm requiring ≥ 3 counts per min and ≥ 2 hours per day, were used to process the accelerometer data.
  
  
  RESULTS
  Processing the accelerometer data with different algorithms resulted in different levels of counts per day, sedentary, and moderate-to-vigorous physical activity. Reliability correlations were very good to excellent (ICC = 0.70-0.90) for almost all algorithms and there were no significant differences between physical activity measures at Time 1 and Time 2.
  
  
  CONCLUSIONS
  This paper presents the first assessment of test-retest reliability of the Actigraph over separate administrations in free-living subjects. The ActiGraph was highly reliable in measuring activity over a 7-day period in natural settings but data were sensitive to the algorithms used to process them.
authors: Dept of Epidemiology and Community Health, University of Minnesota, Minneapolis, MN, USA.
authors: John R
authors: JR
authors: Sirard
authors: Ann
authors: A
authors: Forsyth
authors: J Michael
authors: JM
authors: Oakes
authors: Kathryn H
authors: KH
authors: Schmitz
mesh_terms: D000328:Adult; D000465:Algorithms; D015992:Body Mass Index; D003625:Data Collection / Q000295:instrumentation* / Q000379:methods*; D001330:Electronic Data Processing; D015444:Exercise; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008910:Minnesota; D015203:Reproducibility of Results; D012111:Residence Characteristics; D012959:Socioeconomic Factors