title: Estimating minimally important differences for two vision-specific quality of life measures.
abstract: OBJECTIVE
  To estimate minimally important differences (MIDs) for the Visual Activities Questionnaire (VAQ) and the National Eye Institute-Visual Function Questionnaire (NEI-VFQ).
  
  
  METHODS
  A total of 607 subjects with newly-diagnosed open-angle glaucoma (OAG) was enrolled in the Collaborative Initial Glaucoma Treatment Study (CIGTS) and randomized to initial treatment with medications or surgery. Subjects underwent an ophthalmic examination and telephone-administered quality of life (QOL) interview before randomization and every six months thereafter. The VAQ and NEI-VFQ were used to assess participants' perceptions of their visual function. Clinical measures included the mean deviation (MD) from Humphrey 24-2 full threshold visual field (VF) testing, and best-corrected visual acuity (VA) measured using the Early Treatment Diabetic Retinopathy Study (ETDRS) protocol. Anchor-based (using MD and VA) and distribution-based methods were used to estimate MIDs.
  
  
  RESULTS
  Anchor-based cross-sectional analyses at 66 months follow-up found a 10-letter increment in better eye VA corresponded to MIDs of 5.2 units for VAQ and 3.8 units for NEI-VFQ total scores. A 3-dB increment in the better eye MD yielded MIDs of 2.6 and 2.3 units for the same two questionnaires. In longitudinal analyses, MIDs for the VAQ were 3.2 units for a 10-letter change of VA and 3.4 units for a 3-dB change in the MD. Distribution-based MIDs were larger.
  
  
  CONCLUSIONS
  A range of MIDs for the VAQ (2.6-6.5 units) and NEI-VFQ (2.3-3.8 units) was found. Although similar in magnitude, MIDs were sensitive to the MID estimation method, the anchor chosen, and differences between questionnaires. (ClinicalTrials.gov number, NCT00000149.).
authors.affiliation: Department of Biostatistics, University of Michigan, Ann Arbor, Michigan, United States.
authors.forename: Brenda W
authors.initials: BW
authors.lastname: Gillespie
authors.affiliation: Department of Ophthalmology and Visual Sciences, University of Michigan, Ann Arbor, Michigan, United States Department of Epidemiology, University of Michigan, Ann Arbor, Michigan, United States.
authors.forename: David C
authors.initials: DC
authors.lastname: Musch
authors.affiliation: Department of Ophthalmology and Visual Sciences, University of Michigan, Ann Arbor, Michigan, United States.
authors.forename: Leslie M
authors.initials: LM
authors.lastname: Niziol
authors.affiliation: Department of Health Behavior and Health Education, University of Michigan, Ann Arbor, Michigan, United States.
authors.forename: Nancy K
authors.initials: NK
authors.lastname: Janz
mesh_terms: D000328:Adult; D000368:Aged; D005260:Female; D005500:Follow-Up Studies; D005902:Glaucoma, Open-Angle / Q000503:physiopathology / Q000523:psychology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D011788:Quality of Life; D012189:Retrospective Studies; D011795:Surveys and Questionnaires; D013997:Time Factors; D014792:Visual Acuity / Q000502:physiology*; D014794:Visual Fields / Q000502:physiology*
keywords: CIGTS; MID; NEI-VFQ; VAQ; glaucoma