title: Visual disability variables. I: the importance and difficulty of activity goals for a sample of low-vision patients.
abstract: OBJECTIVE
  To test the validity and reliability of latent trait measures estimated from ratings by low-vision patients of the importance and difficulty of selected activity goals.
  
  
  METHODS
  Validation of a telephone-administered functional assessment instrument using Rasch analysis of self-assessment ratings.
  
  
  METHODS
  Telephone interviews of respondents in their homes. Participants Consecutive series of 600 outpatients with low vision.
  
  
  METHODS
  Not applicable.
  
  
  METHODS
  Ratings of the importance and difficulty of achieving 41 activity goals. Person and item traits were measured with the Andrich rating scale model. Measurement validity and reliability were tested statistically by comparing response patterns and distributions with measurement model expectations.
  
  
  RESULTS
  Patients could distinguish only 3 categories of importance and 4 categories of difficulty. The distributions of person and item measure fit statistics were consistent with 2 unidimensional constructs: value of independence estimated from importance ratings and visual ability estimated from difficulty ratings. However, 8 of 41 activity goals were poor estimators of value of independence and 7 of 41 activity goals were poor estimators of visual ability. Person measure distributions could be divided into 3 statistically distinct strata for estimates from both importance ratings and difficulty ratings. Item measure distributions could be divided into 21 strata for estimates from importance ratings and 7 strata for estimates from difficulty ratings.
  
  
  CONCLUSIONS
  The 2 variables that define visual disability-value of independence and visual ability-are valid constructs that can be estimated accurately and reliably from patient ratings of the importance and difficulty of activity goals.
authors.affiliation: Lions Vision Research and Rehabilitation Center, Wilmer Ophthalmological Institute, The Johns Hopkins University School of Medicine, Baltimore, MD, USA. rmassof@lions.med.jhu.edu
authors.forename: Robert W
authors.initials: RW
authors.lastname: Massof
authors.forename: Cynthia T
authors.initials: CT
authors.lastname: Hsu
authors.forename: Frank H
authors.initials: FH
authors.lastname: Baker
authors.forename: G David
authors.initials: GD
authors.lastname: Barnett
authors.forename: William L
authors.initials: WL
authors.lastname: Park
authors.forename: James T
authors.initials: JT
authors.lastname: Deremeik
authors.forename: Carol
authors.initials: C
authors.lastname: Rainey
authors.forename: Cathy
authors.initials: C
authors.lastname: Epstein
mesh_terms: D000203:Activities of Daily Living; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D004185:Disability Evaluation; D005260:Female; D006040:Goals; D006801:Humans; D008297:Male; D008875:Middle Aged; D011594:Psychometrics; D011795:Surveys and Questionnaires; D013689:Telephone; D015354:Vision, Low / Q000503:physiopathology* / Q000534:rehabilitation