Sample size calculation: Accepting an alfa risk of 0.05 and a beta risk of 0.20 in a two-sided Student's T-test, 50 subjects are necessary in first group and 50 in the second to recognize as statistically significant a difference greater than or equal to 10% in IOP, with a common standard deviation of 17%. A drop-out rate of 8% has been anticipated. If no drop-out rate is considered, the number of needed subjects is 92 (46 per group).