title: Predictors of validity and reliability of a physical activity record in adolescents.
abstract: BACKGROUND
  Poor to moderate validity of self-reported physical activity instruments is commonly observed in young people in low- and middle-income countries. However, the reasons for such low validity have not been examined in detail. We tested the validity of a self-administered daily physical activity record in adolescents and assessed if personal characteristics or the convenience level of reporting physical activity modified the validity estimates.
  
  
  METHODS
  The study comprised a total of 302 adolescents from an urban and rural area in Ecuador. Validity was evaluated by comparing the record with accelerometer recordings for seven consecutive days. Test-retest reliability was examined by comparing registrations from two records administered three weeks apart. Time spent on sedentary (SED), low (LPA), moderate (MPA) and vigorous (VPA) intensity physical activity was estimated. Bland Altman plots were used to evaluate measurement agreement. We assessed if age, sex, urban or rural setting, anthropometry and convenience of completing the record explained differences in validity estimates using a linear mixed model.
  
  
  RESULTS
  Although the record provided higher estimates for SED and VPA and lower estimates for LPA and MPA compared to the accelerometer, it showed an overall fair measurement agreement for validity. There was modest reliability for assessing physical activity in each intensity level. Validity was associated with adolescents' personal characteristics: sex (SED: P=0.007; LPA: P=0.001; VPA: P=0.009) and setting (LPA: P=0.000; MPA: P=0.047). Reliability was associated with the convenience of completing the physical activity record for LPA (low convenience: P=0.014; high convenience: P=0.045).
  
  
  CONCLUSIONS
  The physical activity record provided acceptable estimates for reliability and validity on a group level. Sex and setting were associated with validity estimates, whereas convenience to fill out the record was associated with better reliability estimates for LPA. This tendency of improved reliability estimates for adolescents reporting higher convenience merits further consideration.
authors.affiliation: Nutrition and Child Health Unit, Department of Public Health, Institute of Tropical Medicine, Nationalestraat 155, 2000, Antwerp, Belgium. roosmarijn.verstraeten@ugent.be.
authors.forename: Roosmarijn
authors.initials: R
authors.lastname: Verstraeten
authors.forename: Carl
authors.initials: C
authors.lastname: Lachat
authors.forename: Angélica
authors.initials: A
authors.lastname: Ochoa-Avilés
authors.forename: Maria
authors.initials: M
authors.lastname: Hagströmer
authors.forename: Lieven
authors.initials: L
authors.lastname: Huybregts
authors.forename: Susana
authors.initials: S
authors.lastname: Andrade
authors.forename: Silvana
authors.initials: S
authors.lastname: Donoso
authors.forename: John
authors.initials: J
authors.lastname: Van Camp
authors.forename: Lea
authors.initials: L
authors.lastname: Maes
authors.forename: Patrick
authors.initials: P
authors.lastname: Kolsteren
mesh_terms: D061725:Accelerometry / Q000706:statistics & numerical data; D000293:Adolescent; D002648:Child; D004484:Ecuador; D005260:Female; D006801:Humans; D016014:Linear Models; D008297:Male; D009043:Motor Activity; D015203:Reproducibility of Results; D012424:Rural Population / Q000706:statistics & numerical data; D057566:Self Report; D012737:Sex Factors; D014505:Urban Population / Q000706:statistics & numerical data