title: Assessing change in quality of life using the Oral Health Impact Profile.
abstract: Traditionally, longitudinal studies of oral health have measured only disease progression and ignored improvements in health.
  
  
  OBJECTIVE
  This study examines methodological issues that arise in longitudinal assessment of change in oral health-related quality of life (OHRQOL).
  
  
  METHODS
  Baseline and 2-year follow-up data were used from an observational longitudinal study of 498 people aged 60 years or more living in South Australia. Oral health-related quality of life was measured using the Oral Health Impact Profile (OHIP). Three hypothesized risk predictors (tooth loss, problem-based dental visits and financial hardship) were selected to examine the effects of four methods of measuring change: categorical measures of improvement, deterioration and net change, and a quantitative measure of net change in OHIP scores.
  
  
  RESULTS
  Some 31.7% of people experienced some improvement and 32.7% experienced some deterioration in OHRQOL. All three high-risk groups had approximately twice the rate of deterioration in OHRQOL compared with their corresponding low-risk groups. Surprisingly, high-risk groups also had higher rates of improvement. When measured categorically, these effects did not cancel one another, indicating that improvement and deterioration in OHRQOL can be experienced simultaneously. However, quantitative analyses cause improvements and deteriorations to cancel, and analysis of mean OHIP scores created a spurious impression that change in OHRQOL did not differ between dental visit groups. Furthermore, changes in mean OHIP scores were masked by regression to the mean.
  
  
  CONCLUSIONS
  Oral health-related quality of life measures capture both improvement and deterioration in health status, creating new complexities for conceptualizing and analyzing change in longitudinal studies.
authors.affiliation: Department of Dental Ecology, University of North Carolina at Chapel Hill, 27599-7450, USA. gary_slade@dentistry.unc.edu
authors.forename: G D
authors.initials: GD
authors.lastname: Slade
mesh_terms: D000368:Aged; D000704:Analysis of Variance; D003627:Data Interpretation, Statistical; D003729:Dental Care / Q000191:economics / Q000706:statistics & numerical data; D019456:Health Transition; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D009909:Oral Health; D011203:Poverty; D011788:Quality of Life; D012044:Regression Analysis; D012307:Risk Factors; D018434:Sickness Impact Profile; D013021:South Australia / Q000453:epidemiology; D011795:Surveys and Questionnaires; D016388:Tooth Loss / Q000453:epidemiology