title: The frequency distribution of blood-flow velocities in the extraocular vessels.
abstract: OBJECTIVE
  The study was carried out in order to assess the distribution of normal values of the blood-flow velocity in the extraocular vessels.
  
  
  METHODS
  In 240 healthy visitors to a public fair, blood-flow characteristics in the extraocular vessels were measured, and the resistivity index was calculated. Blood-flow velocity was measured with a color Doppler imaging device, using a 7.5-mHz linear-array transducer. Peak-systolic and end-diastolic blood-flow velocities in the arteries were measured, and the resistivity index was calculated. In the central retinal vein the minimal and maximal blood-flow velocities were measured. The statistical analysis of the 14 measured and calculated variables included descriptive statistics, frequency distribution, and quantile plots.
  
  
  RESULTS
  The quantile plots of the cumulative frequency showed that none of these 14 variables are normally distributed. Also, no normal distribution could be achieved by adjustment of the data by age.
  
  
  CONCLUSIONS
  The blood-flow velocities in the extraocular vessels measured are not distributed normally. Therefore, nonparametric tests are to be used for statistical analysis if the sample size is small. The estimation of tolerance intervals has to be based on distribution-free assumptions.
authors.affiliation: University Eye Clinic, Basel, Switzerland.
authors.forename: H J
authors.initials: HJ
authors.lastname: Kaiser
authors.forename: A
authors.initials: A
authors.lastname: Schoetzau
authors.forename: J
authors.initials: J
authors.lastname: Flammer
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001783:Blood Flow Velocity; D002648:Child; D002924:Ciliary Body / Q000098:blood supply; D005123:Eye / Q000098:blood supply*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009880:Ophthalmic Artery / Q000000981:diagnostic imaging / Q000502:physiology; D012016:Reference Values; D012161:Retinal Artery / Q000000981:diagnostic imaging / Q000502:physiology; D012169:Retinal Vein / Q000000981:diagnostic imaging / Q000502:physiology; D018615:Ultrasonography, Doppler, Color