EVALUATING AND REPORTING ASTIGMATISM FOR INDIVIDUAL AND AGGREGATE DATA

Purpose: To demonstrate the proper method for evaluating and reporting astigma tism for individual and aggregate data. Setting: University o f Texas Medical School and Cullen Eye Institute, Baylor College of Med icine, Houston, Texas, USA. Methods: The surgically induced refractive change (SIRC) was determined for three data sets of patients who have had keratorefractive (photorefractive keratectomy) or cataract surger y. To make changes in refraction comparable, vertex distances for the refractions and keratometric index of refraction were considered. Doub led-angle plots and single-angle plots were then used to display the d ata. Polar values (cylinder and axis) were converted to a Cartesian (x and y) coordinate system to determine the mean value of the induced a stigmatism for each data set. Results: Doubled-angle plots clearly dem onstrated the trends of induced astigmatism for each data set, and the mean value for induced astigmatism agreed exactly with the intuitive appearance of the plot. Conclusions: Converting astigmatism data to a Cartesian coordinate system allowed the correct computation of descrip tive statistics such as mean values, standard deviations, and correlat ion coefficients. Using doubled-angle plots to display the data provid es the investigator with the best method of recognizing trends in the data.