The dimensionality of memory strength between levels of both serial position and word frequency category

In recognition tasks where Signal Detection Theory (SDT) is applied, a suitable decision variable is required. This variable is typically assumed to be a global assessment of memory strength. A further assumption is that factors such as old-new, serial position (SP), and word frequency (WF) simply act to change the value of strength across levels within the factor. In terms of SDT, the decision variable is assumed to be uni-dimensional. Four experiments are reported that provide a direct empirical assessment of this assumption for SP and WF. Judgements of items (JOI), recency (JOR), and primacy (JOP) were measured for SP; and subjective frequency (JOF) was measured for WE Multiple dimensions imply non-additive effects across factor levels and different retrieval processes or the influence of an extra process. A single dimension is consistent with an additive effect of factor levels and current theorizing. The data are in fairly good agreement with the unidimensional assumption across all four factors with the exception of JOR and JOP. These showed less uni-dimensionality for comparisons between beginning vs. end list items. The implications of this are discussed.