The effect of age on human corneal thickness. Statistical implications of power analysis

The corneal thickness of 108 human subjects, ranging from 17 to 75 years of age, was measured using ultrasound pachometry. One central, four mid-peripheral and four peripheral corneal positions along the vertical and horizontal meridians were assessed using ultrasound pachometry. No significant differences were found in the thicknesses of the central, midperipheral or peripheral cornea with increasing age using analysis of variance. These results suggest that ageing has no significant effect on human corneal thickness between the ages of 16 to 75 years. However the high Type II error probability (beta = 0.90) suggests that 108 subjects (18 in each age group) were insufficient to adequately answer the question. Thus power analysis may help explain the conflicting reports available in literature. The diversity in data interpretation may be due to the statistically small sample sizes used in most studies. Power analysis shows that at least 80 subjects are needed in each age group (480 subjects in total) before a statistically reliable test of the null hypothesis is possible. This study emphasizes the importance of power analysis in calculating an adequate sample size.