title: Latent variable mixture models: a promising approach for the validation of patient reported outcomes.
abstract: OBJECTIVE
  A fundamental assumption of patient-reported outcomes (PRO) measurement is that all individuals interpret questions about their health status in a consistent manner, such that a measurement model can be constructed that is equivalently applicable to all people in the target population. The related assumption of sample homogeneity has been assessed in various ways, including the many approaches to differential item functioning analysis.
  
  
  METHODS
  This expository paper describes the use of latent variable mixture modeling (LVMM), in conjunction with item response theory (IRT), to examine: (a) whether a sample is homogeneous with respect to a unidimensional measurement model, (b) implications of sample heterogeneity with respect to model-predicted scores (theta), and (c) sources of sample heterogeneity. An example is provided using the 10 items of the Short-Form Health Status (SF-36(®)) physical functioning subscale with data from the Canadian Community Health Survey (2003) (N = 7,030 adults in Manitoba).
  
  
  RESULTS
  The sample was not homogeneous with respect to a unidimensional measurement structure. Specification of three latent classes, to account for sample heterogeneity, resulted in significantly improved model fit. The latent classes were partially explained by demographic and health-related variables.
  
  
  CONCLUSIONS
  The illustrative analyses demonstrate the value of LVMM in revealing the potential implications of sample heterogeneity in the measurement of PROs.
authors.affiliation: School of Nursing, Trinity Western University, 7600 Glover Rd, Langley, BC, Canada. rick.sawatzky@twu.ca
authors.forename: Richard
authors.initials: R
authors.lastname: Sawatzky
authors.forename: Pamela A
authors.initials: PA
authors.lastname: Ratner
authors.forename: Jacek A
authors.initials: JA
authors.lastname: Kopec
authors.forename: Bruno D
authors.initials: BD
authors.lastname: Zumbo
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002170:Canada; D002648:Child; D005260:Female; D006304:Health Status; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D017063:Outcome Assessment, Health Care / Q000379:methods*; D010361:Patients; D011594:Psychometrics; D011795:Surveys and Questionnaires / Q000592:standards*; D055815:Young Adult