A comparison of methods for measuring the interletter similarity between capital letters

Measures of interletter similarity are often required in perception experiments. The most reliable and valid of the available measures appears to be Townsend’s (1971) set of similarity parameters based on the Luce choice model. A simple mechanical measure offered a fairly strong prediction of the Luce choice-model similarity measure, as did a subjective rating measure based on the 10-point visual similarity ratings of eight subjects. By comparison, Gibson et al.’s (1963) matching-confusion matrix faired poorly, as did Gibson’s (1969) distinctive feature analysis based on a letter pair’s number of shared features. Distinctive feature analysis was significantly improved by substituting the feature set proposed by Geyer and DeWald (1973) or by weighting the features optimally via regression analysis. Such analyses suggested that figural curvature may be a particularly important perceptual feature, but in no case did these feature-analytic models predict the Luce measure as well as the mechanical or subjective rating measures.