title: Examining the relationship between health-related quality of life and increasing numbers of diagnoses.
abstract: OBJECTIVE
  Little is known about estimating utilities for comorbid (or 'joint') health states. Several joint health state prediction models have been suggested (for example, additive, multiplicative, best-of-pair, worst-of-pair, etc.), but no general consensus has been reached. The purpose of the study is to explore the relationship between health-related quality of life (HRQoL) and increasing numbers of diagnoses.
  
  
  METHODS
  We analyzed a large dataset containing respondents' ICD-9 diagnoses and preference-based HRQoL (EQ-5D and SF-6D). Data were stratified by the number of diagnoses, and mean HRQoL values were estimated. Several adjustments, accounting for the respondents' age, sex, and the severity of the diagnoses, were carried out. Our analysis fitted additive and multiplicative models to the data and assessed model fit using multiple standard model selection methods.
  
  
  RESULTS
  A total of 39,817 respondents were included in the analyses. Average HRQoL values were represented well by both linear and multiplicative models. Although results across all analyses were similar, adjusting for severity of diagnoses, age, and sex strengthened the linear model's performance measures relative to the multiplicative model. Adjusted R (2) values were above 0.99 for all analyses (i.e., all adjusted analyses, for both HRQoL instruments), indicating a robust result.
  
  
  CONCLUSIONS
  Additive and multiplicative models perform equally well within our analyses. A practical implication of our findings, based on the presumption that a linear model is simpler than an additive model, is that an additive model should be preferred unless there is compelling evidence to the contrary.
authors.affiliation: Health Services Research Center, Akershus University Hospital, Postboks 1000, 1478, Lørenskog, Akershus, Norway. mathias.barra@ahus.no.
authors.forename: Mathias
authors.initials: M
authors.lastname: Barra
authors.affiliation: Health Services Research Center, Akershus University Hospital, Postboks 1000, 1478, Lørenskog, Akershus, Norway.|Department of Health Management and Health Economics, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Liv Ariane
authors.initials: LA
authors.lastname: Augestad
authors.affiliation: Faculty of Health Sciences, Simon Fraser University, Burnaby, BC, Canada.|Centre for Clinical Epidemiology and Evaluation, Vancouver Coastal Health Research Institute, Vancouver, BC, Canada.
authors.forename: David G T
authors.initials: DG
authors.lastname: Whitehurst
authors.affiliation: Health Services Research Center, Akershus University Hospital, Postboks 1000, 1478, Lørenskog, Akershus, Norway.|Department of Health Management and Health Economics, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Kim
authors.initials: K
authors.lastname: Rand-Hendriksen
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015897:Comorbidity; D005260:Female; D006304:Health Status; D006801:Humans; D038801:International Classification of Diseases; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D011788:Quality of Life; D011795:Surveys and Questionnaires; D055815:Young Adult
keywords: Comorbidity; EQ-5D; Health-related quality of life; Health-state utility value; SF-6D