As operation of complicated systems becomes an everyday affair, cognitive complexity that the user experiences becomes an important issue in designing those systems. To examine user complexity, several models such as Task Action Grammar(TAG), General Transition Network(GTN), and Goals-Operators-Methods-Selection-rules (GOMS) have been suggested. TAG is a formal representation of user's task knowledge of how to carry out tasks interfacing devices. TAG uses the number of rule schemas as the measure of user's cognitive complexity. It is reasonable to say that more consistent user interface may require less number of rule schemas.
This paper hypothesize that the user's cognitive complexity does not merely depend on the number of rule schemas but also on the homogeneity of simple tasks grouped in same rule schemas. Especiually, a salient elementary operation may lead the user to recognize the action sequences that include the operation as a natural group. This hypothesis was well verified by an experiment using alternative interfaces of an ultrasound diagnosis device. Therefore, not only the number of rule schemas but also how they are grouped is shown to be important.
