title: When to use agreement versus reliability measures.
abstract: BACKGROUND
  Reproducibility concerns the degree to which repeated measurements provide similar results. Agreement parameters assess how close the results of the repeated measurements are, by estimating the measurement error in repeated measurements. Reliability parameters assess whether study objects, often persons, can be distinguished from each other, despite measurement errors. In that case, the measurement error is related to the variability between persons. Consequently, reliability parameters are highly dependent on the heterogeneity of the study sample, while the agreement parameters, based on measurement error, are more a pure characteristic of the measurement instrument.
  
  
  RESULTS
  Using an example of an interrater study, in which different physical therapists measure the range of motion of the arm in patients with shoulder complaints, the differences and relationships between reliability and agreement parameters for continuous variables are illustrated.
  
  
  CONCLUSIONS
  If the research question concerns the distinction of persons, reliability parameters are the most appropriate. But if the aim is to measure change in health status, which is often the case in clinical practice, parameters of agreement are preferred.
authors.affiliation: Institute for Research in Extramural Medicine, VU University Medical Center, Amsterdam, Van der Boechorststraat 7, Amsterdam 1081 BT, The Netherlands. hcw.devet@vumc.nl
authors.forename: Henrica C W
authors.initials: HC
authors.lastname: de Vet
authors.forename: Caroline B
authors.initials: CB
authors.lastname: Terwee
authors.forename: Dirk L
authors.initials: DL
authors.lastname: Knol
authors.forename: Lex M
authors.initials: LM
authors.lastname: Bouter
mesh_terms: D006801:Humans; D015588:Observer Variation; D017063:Outcome Assessment, Health Care / Q000379:methods / Q000592:standards; D026741:Physical Therapy Modalities; D016059:Range of Motion, Articular; D015203:Reproducibility of Results; D012107:Research Design; D020069:Shoulder Pain / Q000503:physiopathology / Q000534:rehabilitation; D009626:Terminology as Topic