The selectivity of preparation.

This paper examines the extent to which preparatory processes are selective (a) between two performance measures, speed and accuracy, (b) within a processing stage, for example, stimulus encoding and response execution, and (c) among possible stimulus presentation times. It is shown that the appearance of selectivity between speed and accuracy depends on whether these variables are considered over the possible preparatory states on a given trial, over the trials within an experimental condition, or over different conditions, and the relevance of this result to the measurement of processing capacity is discussed. Selectivity within processing stages is demonstrated by applying a parametric processing model to data from letter-matching tasks in which the identity of the first letter is probabilistically related to that of the second letter. In these tasks, reaction time differences between physical and name matches and between same and different responses depend on the stimulus and response contingencies and on the stimulus presentation times. Finally, a method is presented for resolving observed error rates into components that reflect the accuracy of each processing stage.