title: Systematic review of the validity and reliability of consumer-wearable activity trackers.
abstract: BACKGROUND
  Consumer-wearable activity trackers are electronic devices used for monitoring fitness- and other health-related metrics. The purpose of this systematic review was to summarize the evidence for validity and reliability of popular consumer-wearable activity trackers (Fitbit and Jawbone) and their ability to estimate steps, distance, physical activity, energy expenditure, and sleep.
  
  
  METHODS
  Searches included only full-length English language studies published in PubMed, Embase, SPORTDiscus, and Google Scholar through July 31, 2015. Two people reviewed and abstracted each included study.
  
  
  RESULTS
  In total, 22 studies were included in the review (20 on adults, 2 on youth). For laboratory-based studies using step counting or accelerometer steps, the correlation with tracker-assessed steps was high for both Fitbit and Jawbone (Pearson or intraclass correlation coefficients (CC) > =0.80). Only one study assessed distance for the Fitbit, finding an over-estimate at slower speeds and under-estimate at faster speeds. Two field-based studies compared accelerometry-assessed physical activity to the trackers, with one study finding higher correlation (Spearman CC 0.86, Fitbit) while another study found a wide range in correlation (intraclass CC 0.36-0.70, Fitbit and Jawbone). Using several different comparison measures (indirect and direct calorimetry, accelerometry, self-report), energy expenditure was more often under-estimated by either tracker. Total sleep time and sleep efficiency were over-estimated and wake after sleep onset was under-estimated comparing metrics from polysomnography to either tracker using a normal mode setting. No studies of intradevice reliability were found. Interdevice reliability was reported on seven studies using the Fitbit, but none for the Jawbone. Walking- and running-based Fitbit trials indicated consistently high interdevice reliability for steps (Pearson and intraclass CC 0.76-1.00), distance (intraclass CC 0.90-0.99), and energy expenditure (Pearson and intraclass CC 0.71-0.97). When wearing two Fitbits while sleeping, consistency between the devices was high.
  
  
  CONCLUSIONS
  This systematic review indicated higher validity of steps, few studies on distance and physical activity, and lower validity for energy expenditure and sleep. The evidence reviewed indicated high interdevice reliability for steps, distance, energy expenditure, and sleep for certain Fitbit models. As new activity trackers and features are introduced to the market, documentation of the measurement properties can guide their use in research settings.
authors.affiliation: Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina-Chapel Hill, 137 East Franklin Street, Suite 306, Chapel Hill, 27514, NC, USA. kelly_evenson@unc.edu.|RTI International, Research Triangle Park, NC, USA. kelly_evenson@unc.edu.
authors.forename: Kelly R
authors.initials: KR
authors.lastname: Evenson
authors.affiliation: Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina-Chapel Hill, 137 East Franklin Street, Suite 306, Chapel Hill, 27514, NC, USA. mgoto@live.unc.edu.
authors.forename: Michelle M
authors.initials: MM
authors.lastname: Goto
authors.affiliation: RTI International, Research Triangle Park, NC, USA. rfurberg@rti.org.
authors.forename: Robert D
authors.initials: RD
authors.lastname: Furberg
mesh_terms: D061725:Accelerometry / Q000295:instrumentation* / Q000379:methods; D004734:Energy Metabolism; D005260:Female; D006801:Humans; D008297:Male; D018670:Monitoring, Ambulatory / Q000295:instrumentation* / Q000379:methods; D015203:Reproducibility of Results; D012420:Running; D012890:Sleep; D054928:Validation Studies as Topic; D016138:Walking