[Probability studies on the variations of anterior corneal astigmatism]

The probability method of analysis of astigmatism variations respects both astigmatism components, quantity and axis. The two random variable model that we introduce here analyses cylinder variations and axial variations. After describing the contingency table and the probability table. Expectation of each component and its confidence interval are calculated together with the variance and the standard deviation. Covariance and correlation are calculated. If we want to compare different groups we test cylinders with cylinders and axes with axes. This method does not distort reality and is applicable to the semi meridians from 0 degree to 360 degrees. Only large samples of patients can be studied by this method.