title: The relationship between variability and sensitivity in large-scale longitudinal visual field data.
abstract: OBJECTIVE
  Evaluation of progressive visual field (VF) damage is often based on pointwise sensitivity data from standard automated perimetry; however, frequency-of seeing and test-retest studies demonstrate that these measurements can be highly variable, especially in areas of damage. The aim of this study was to characterize VF variability by the level of sensitivity using a statistical method to quantify heteroscedasticity.
  
  
  METHODS
  A total of 14,887 Humphrey 24-2 SITA Standard VFs from 2736 patients (2736 eyes) attending Moorfields Eye Hospital from 1997 to 2009 were studied retrospectively. The VF series of each eye was analyzed using pointwise linear regression of sensitivity over time, with residuals (difference from fitted-value) from each regression pooled according to both observed and fitted sensitivities.
  
  
  RESULTS
  The median (interquartile range) patient age, follow-up, and series length was 64 (54-71) years, 5.5 (3.9-7.0) years, and 6 (5-7) VFs, respectively. The inferred variability as a function of fitted-sensitivity was in good agreement with previous estimates. Variability was also described as a function of measured sensitivity, which confirmed that variability increased rapidly as the observed sensitivity decreased.
  
  
  CONCLUSIONS
  This study highlights a new approach for characterizing VF variability by the level of sensitivity. A considerable strength of the method is that inference is based on thousands of clinic patients rather than the tens of subjects in test-retest studies. The results can help distinguish real VF progression from measurement variability and will be used in models for glaucoma progression detection.
authors.affiliation: Department of Optometry and Visual Science, City University London, UK.
authors.forename: Richard A
authors.initials: RA
authors.lastname: Russell
authors.forename: David P
authors.initials: DP
authors.lastname: Crabb
authors.forename: Rizwan
authors.initials: R
authors.lastname: Malik
authors.forename: David F
authors.initials: DF
authors.lastname: Garway-Heath
mesh_terms: D000368:Aged; D000465:Algorithms; D018450:Disease Progression; D005500:Follow-Up Studies; D005901:Glaucoma / Q000175:diagnosis*; D006801:Humans; D008875:Middle Aged; D015588:Observer Variation; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D012684:Sensory Thresholds; D014786:Vision Disorders / Q000175:diagnosis*; D058609:Visual Field Tests; D014794:Visual Fields