title: Correcting bias in self-rated quality of life: an application of anchoring vignettes and ordinal regression models to better understand QoL differences across commuting modes.
abstract: OBJECTIVE
  Likert scales are frequently used in public health research, but are subject to scale perception bias. This study sought to explore scale perception bias in quality-of-life (QoL) self-assessment and assess its relationships with commuting mode in the Sydney Travel and Health Study.
  
  
  METHODS
  Multilevel ordinal logistic regression analysis was used to analyse the association between two global QoL items about overall QoL and health satisfaction, with usual travel mode to work or study. Anchoring vignettes were applied using parametric and simpler nonparametric methods to detect and adjust for differences in reporting behaviour across age, sex, education, and income groups.
  
  
  RESULTS
  The anchoring vignettes exposed differences in scale responses across demographic groups. After adjusting for these biases, public transport users (OR = 0.37, 95 % CI 0.21-0.65), walkers (OR = 0.44, 95 % CI 0.24-0.82), and motor vehicle users (OR = 0.47, 95 % CI 0.25-0.86) were all found to have lower odds of reporting high QoL compared with bicycle commuters. Similarly, the odds of reporting high health satisfaction were found to be proportionally lower amongst all competing travel modes: motor vehicle users (OR = 0.31, 95 % CI 0.18-0.56), public transport users (OR = 0.34, 95 % CI 0.20-0.57), and walkers (OR = 0.35, 95 % CI 0.20-0.64) when compared with cyclists. Fewer differences were observed in the unadjusted models.
  
  
  CONCLUSIONS
  Application of the vignettes by the two approaches removed scaling biases, thereby improving the accuracy of the analyses of the associations between travel mode and quality of life. The adjusted results revealed higher quality of life in bicycle commuters compared with all other travel mode users.
authors.affiliation: Prevention Research Collaboration, Sydney School of Public Health, University of Sydney, Sydney, NSW, 2006, Australia. melanie.crane@sydney.edu.au.
authors.forename: Melanie
authors.initials: M
authors.lastname: Crane
authors.affiliation: Prevention Research Collaboration, Sydney School of Public Health, University of Sydney, Sydney, NSW, 2006, Australia.
authors.forename: Chris
authors.initials: C
authors.lastname: Rissel
authors.affiliation: Institute of Transport and Logistics Studies, The University of Sydney Business School, University of Sydney, Sydney, NSW, 2006, Australia.
authors.forename: Stephen
authors.initials: S
authors.lastname: Greaves
authors.affiliation: Centre for Chronic Disease Prevention, College of Public Health, Medical and Veterinary Sciences, James Cook University, Cairns, QLD, 4870, Australia.
authors.forename: Klaus
authors.initials: K
authors.lastname: Gebel
mesh_terms: D000293:Adolescent; D000328:Adult; D015982:Bias; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D011788:Quality of Life / Q000523:psychology*; D014186:Transportation / Q000706:statistics & numerical data*; D055815:Young Adult
keywords: Anchoring vignettes; Commuting; Cycling; Differential item functioning; Ordinal logistic regression; Quality of life