title: The retest distribution of the visual field summary index mean deviation is close to normal.
abstract: When modelling optimum strategies for how best to determine visual field progression in glaucoma, it is commonly assumed that the summary index mean deviation (MD) is normally distributed on repeated testing. Here we tested whether this assumption is correct.
  
  
  
  We obtained 42 reliable 24-2 Humphrey Field Analyzer SITA standard visual fields from one eye of each of five healthy young observers, with the first two fields excluded from analysis. Previous work has shown that although MD variability is higher in glaucoma, the shape of the MD distribution is similar to that found in normal visual fields. A Shapiro-Wilks test determined any deviation from normality. Kurtosis values for the distributions were also calculated.
  
  
  
  Data from each observer passed the Shapiro-Wilks normality test. Bootstrapped 95% confidence intervals for kurtosis encompassed the value for a normal distribution in four of five observers. When examined with quantile-quantile plots, distributions were close to normal and showed no consistent deviations across observers.
  
  
  
  The retest distribution of MD is not significantly different from normal in healthy observers, and so is likely also normally distributed - or nearly so - in those with glaucoma. Our results increase our confidence in the results of influential modelling studies where a normal distribution for MD was assumed.
authors: Department of Optometry and Vision Sciences, The University of Melbourne, Melbourne, Australia.
authors: Andrew J
authors: AJ
authors: Anderson
authors: Department of Optometry and Vision Sciences, The University of Melbourne, Melbourne, Australia.
authors: Allan C Y
authors: AC
authors: Cheng
authors: Department of Optometry and Vision Sciences, The University of Melbourne, Melbourne, Australia.
authors: Samantha
authors: S
authors: Lau
authors: Department of Optometry and Vision Sciences, The University of Melbourne, Melbourne, Australia.
authors: Anne
authors: A
authors: Le-Pham
authors: Department of Optometry and Vision Sciences, The University of Melbourne, Melbourne, Australia.
authors: Victor
authors: V
authors: Liu
authors: Department of Optometry and Vision Sciences, The University of Melbourne, Melbourne, Australia.
authors: Farahnaz
authors: F
authors: Rahman
mesh_terms: D000328:Adult; D000465:Algorithms; D018450:Disease Progression; D005260:Female; D005901:Glaucoma / Q000175:diagnosis; D006801:Humans; D008297:Male; D015203:Reproducibility of Results; D012684:Sensory Thresholds / Q000502:physiology; D014786:Vision Disorders / Q000175:diagnosis*; D058609:Visual Field Tests / Q000379:methods*; D014794:Visual Fields / Q000502:physiology*; D055815:Young Adult
keywords: glaucoma; linear regression; mean deviation; progression; robust regression; visual field