title: Technical Note: grading the vertical cup:disc ratio and the effect of scaling.
abstract: OBJECTIVE
  To evaluate the effect of scaling on sensitivity to change for grading the vertical cup:disc ratio (CDR).
  
  
  METHODS
  Vertical CDR was assessed by six observers (three ophthalmologists and three optometrists) on 43 stereo disc photographs. Repeated observations were made for both 0.1 and 0.05 interval scales. Paired differences were calculated for all observers and each observer separately. Mean and standard deviation of differences and agreement statistics were used to compare scales.
  
  
  RESULTS
  Five observers demonstrated a reduction in the spread of differences (mean difference 0.19 to 0.15) and all observers demonstrated a reduction in concordance using the finer scale (mean concordance 54% to 39%).
  
  
  CONCLUSIONS
  The use of a finer scale reduces test-retest variability and increases sensitivity to change when estimating the vertical CDR. Use of this scale does not require any additional resource and it may be easily implemented in routine clinical practice.
authors.affiliation: Ophthalmology Department, Nottingham University Hospitals NHS Trust, Nottingham NG7 2UH, UK.
authors.forename: Ruth
authors.initials: R
authors.lastname: Bennett
authors.forename: Paul G D
authors.initials: PG
authors.lastname: Spry
authors.forename: Cecilia H
authors.initials: CH
authors.lastname: Fenerty
authors.forename: Robert A
authors.initials: RA
authors.lastname: Harper
mesh_terms: D002983:Clinical Competence / Q000592:standards*; D016001:Confidence Intervals; D003941:Diagnostic Techniques, Ophthalmological; D005901:Glaucoma / Q000175:diagnosis*; D006801:Humans; D015588:Observer Variation; D009898:Optic Disk / Q000473:pathology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity