The discovery of processing stages: Extensions of Donders' method

A new method is proposed for using reaction-time (RT) measurements to study stages of information processing. It overcomes limitations of Donders' and more recent methods, and permits the discovery of stages, assessment of their properties, and separate testing of the additivity and stochastic independence of stage durations. The main feature of the additive-factor method is the search for non-interacting effects of experimental factors on mean RT. The method is applied to several binary-classification experiments, where it leads to a four-stage model, and to an identification experiment, where it distinguishes two stages. The sets of stages inferred from both these and other data are shown to carry substantive implications. It is demonstrated that stage-durations may be additive without being stochastically independent, a result that is relevant to the formulation of mathematical models of RT.