Shall We Count Numbers of Eyes or Numbers of Subjects?

The number of patients or animals studied is an important criterion for critically evaluating biomedical research. With other factors equal, results of a study of 40 subjects are more "solid" than those of a study of only 20. That is, a larger study provides more information than a smaller study. Or, in other words, estimates of parameters such as the mean, standard deviation, or the proportion with a given characteristic generally have greater precision when derived from a larger sample. The number of subjects is usually denoted by the letter n. In ophthalmic investigations is n the number of eyes or the number of subjects? Published vision research papers often give either the number of eyes studied and not the number of subjects, or the number of subjects and not the number of eyes. The purpose of this communication is to explain the need not only for giving both numbers,