The frequency distribution of blood-flow velocities in the extraocular vessels

Purpose: 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.