An Experimental and Theoretical Investigation of the Constant-Ratio Rule and Other Models of Visual Letter Confusion

The constant-ratio rule (CRR) and four interpretations of R. D. Lute’s (In R. D. Lute, R. R. Bush, & E. Galanter (Eds.), Handbook ofmathematicalpsychology (Vol. 1). New York: Wiley, 1963) similarity choice model (SCM) were tested using an alphabetic confusion paradigm. Four stimulus conditions were employed that varied in set size (three, four or five stimulus elements) and set constituency (block letters: A, E, X, F, H, X, A, E, F, H; A. E, F, H, X), and were presented to each subject in independent blocks. The four interpretations of the SCM were generated by constraining one, both, or neither of its similarity and bias parameter sets to be invariant in across-stimulus set model predictions. The strictest interpretation of the SCM (both the similarity and bias parameters constrained), shown to be a special case of the CRR, and the CRR produced nearly equivalent across-set predictions that provided a reasonable first approximation to the data. However, they proved inferior to the least strict SCM (neither the similarity nor bias parameters were constrained; the common interpretation of the SCM in visual confusion). Additionally, the least strict SCM was compared to J. T. Townsend’s (Perception and Psychophysics, 1971, 9, 4&50, 4499454) overlap model, the all-or-none model (J. T. Townsend, Journal of Mathematical Psychology, 1978, 18, 25-38) and a modified version of L. H. Nakatani’s (Journal of Mathematical Psychology, 1972, 9, 104-127) confusion-choice model. Both the least strict SCM and confusion-choice models produced nearly equivalent within stimulus set predictions that were superior to the overlap and all-or-none within-set predictions. Measurement conditions related to model structure and equivalence relations among the models, many of them new, were examined and compared with the statistical fit results of the investigation.