Notes on the Statistical Treatment of the Visual Resolution

The usual visual acuity scales are not interval scales; they are not isomorphic to the numerical system known as arithmetic. It is not permissible to manipulate the scores by adding and dividing, which are necessary operations in finding mean, variance and standard deviation. The resulting statistical difficulties are discussed and some methods of calculation are presented. It would, however, be preferable if the measurements could be performed with a chart graduated in equal steps in terms of minimum separabile. The normal distribution is discussed in its application to visual resolution. The relations between the visual resolution and some other variables are discussed.