Frequency distribution of IOP. Analysis of a material using the gamma distribution

The results of a population-based (age greater than or equal to 65 years) examination of the frequency distribution of IOP are reported. 3726 eyes in 1887 individuals were included in the examination. By consistent use of conventional, well-defined criteria, glaucoma was diagnosed in 6.4% of the individual eyes. In the total material, the average IOP was 18.0 mmHg. Average IOP was 28.7 mmHg in eyes with glaucomatous damage, vs 17.2 mmHg in the normal eyes. Variance of IOP was much higher in the glaucoma group as compared with the normal group. The IOP frequency distribution curve did not fit with a Gaussian curve; it was asymmetric, skewed to the right. The distribution curve obtained by plotting the recorded frequencies was disturbed by irrelevant irregularities, both random and systematic. The irregularities could be evened out by the application of regression analysis, using the equation for the gamma distribution. The IOP frequency distribution is well represented by a gamma distribution curve. The fit is improved by excluding eyes with extremely high and low IOP.