We present a methodology and a tool for analyzing the activity of an operator interacting with a complex technical device. The goal is of refining cognitive models of the operator by relating them to patterns of behavior in real situations. The activity is observed to be modeled as a trace having a graph structure. The trace is transformed according to a "use model" in order to become meaningful in the context of modeling theories. Our "Trace Based System" thus gathers both a representation of the activity and of the analyst's expertise for facilitating the discovery of knowledge in the field of cognitive psychology. The approach is illustrated by its application for car driver cognitive modeling.