TODAM and the list-strength and list-length effects: comment on Murdock and Kahana (1993a)

B. B. Murdock and M. J. Kahana (1993a) presented a continuous memory version of the theory of distributed associative memory (TODAM) model; they claimed that this model predicts list-strength and list-length findings, including those reported by R. Ratcliff, S. E. Clark, and R. M. Shiffrin (1990) and K. Murnane and R. M. Shiffrin (1991a). This model is quite similar to one discussed by R. M. Shiffrin, R. Ratcliff, and S. Clark (1990), who rejected the model on the basis of its inability to predict both an absent or negative list-strength effect (when strength is varied by repetitions) and a present list-length effect. In this comment we elaborate the earlier discussion and demonstrate that the version of TODAM proposed by B. B. Murdock and M. J. Kahana (1993a) indeed fails for this reason. We show this first for a somewhat simplified version of the model for which derivations are obvious and then in a simulation of the complete version using the parameter values suggested by B. B. Murdock and M. J. Kahana (1993a).