Refractive variation during autorefraction: multivariate distribution of refractive status.

The refractive variation of a sample of 106 university students (63 females and 43 males) studying optometry was examined by means of autorefraction. Stereo-pair scatter plots in Euclidean three-dimensional h-space are used to illustrate the nature of the spread or distribution of data measurements found in particular subjects. A wide variety of different distributions were observed ranging from tightly to loosely clustered arrangements of measurements. Some aspects of departure from multivariate normality, including outliers (atypical measurements in a sample) and polymodal or multimodal distributions, are demonstrated. Outliers appear to be possible anywhere in the space of the scatter plots, although outliers may be more common in the region of h-space corresponding to transitory increases in accommodation. Multimodal distributions may be indicative of changes in ocular fixation during autorefraction or may reflect accommodative or other anomalies. Other departures from multivariate normality such as kurtosis and skewness are also of importance when attempting to form an understanding of variation of refractive state. Measurements made on an artificial or test eye showed very tight clusters in h-space. This suggests that the autorefractor itself contributes little to the variation observed during autorefraction of an eye.