title: Estimation of physical activity levels using cell phone questionnaires: a comparison with accelerometry for evaluation of between-subject and within-subject variations.
abstract: BACKGROUND
  Physical activity promotes health and longevity. Further elaboration of the role of physical activity for human health in epidemiological studies on large samples requires accurate methods that are easy to use, cheap, and possible to repeat. The use of telecommunication technologies such as cell phones is highly interesting in this respect. In an earlier report, we showed that physical activity level (PAL) assessed using a cell phone procedure agreed well with corresponding estimates obtained using the doubly labeled water method. However, our earlier study indicated high within-subject variation in relation to between-subject variations in PAL using cell phones, but we could not assess if this was a true variation of PAL or an artifact of the cell phone technique.
  
  
  OBJECTIVE
  Our objective was to compare within- and between-subject variations in PAL by means of cell phones with corresponding estimates using an accelerometer. In addition, we compared the agreement of daily PAL values obtained using the cell phone questionnaire with corresponding data obtained using an accelerometer.
  
  
  METHODS
  PAL was measured both with the cell phone questionnaire and with a triaxial accelerometer daily during a 2-week study period in 21 healthy Swedish women (20 to 45 years of age and BMI from 17.7 kg/m² to 33.6 kg/m²). The results were evaluated by fitting linear mixed effect models and descriptive statistics and graphs.
  
  
  RESULTS
  With the accelerometer, 57% (95% confidence interval [CI] 40%-66%) of the variation was within subjects, while with the cell phone, within-subject variation was 76% (95% CI 59%-83%). The day-to-day variations in PAL observed using the cell phone questions agreed well with the corresponding accelerometer results.
  
  
  CONCLUSIONS
  Both the cell phone questionnaire and the accelerometer showed high within-subject variations. Furthermore, day-to-day variations in PAL within subjects assessed using the cell phone agreed well with corresponding accelerometer values. Consequently, our cell phone questionnaire is a promising tool for assessing levels of physical activity. The tool may be useful for large-scale prospective studies.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Christin
authors.initials: C
authors.lastname: Bexelius
authors.forename: Sven
authors.initials: S
authors.lastname: Sandin
authors.forename: Ylva
authors.initials: Y
authors.lastname: Trolle Lagerros
authors.forename: Jan-Eric
authors.initials: JE
authors.lastname: Litton
authors.forename: Marie
authors.initials: M
authors.lastname: Löf
mesh_terms: D000328:Adult; D015992:Body Mass Index; D001835:Body Weight; D002153:Calorimetry, Indirect; D040421:Cell Phone / Q000706:statistics & numerical data*; D003625:Data Collection / Q000295:instrumentation / Q000379:methods*; D004734:Energy Metabolism; D005260:Female; D006801:Humans; D008875:Middle Aged; D009043:Motor Activity; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D011795:Surveys and Questionnaires / Q000592:standards*; D013548:Sweden; D014867:Water; D055815:Young Adult
chemical_list: D014867:Water