system and environment state, each crew member's attention allocation, system control inputs, a list of significant events, assessed situations, a procedural time line denoting the procedures being executed by each crew member, a message time line indicating communication traffic and auditory signals, and a milestone time line showing important milestones and events as defined by the modeler.

Recent variants of SAMPLE are written in C++. They run on an IBM PC under the Windows 95 operating system.

SAMPLE has a limited programming environment, but work is under way on developing a graphical editor for model specifications.

SAMPLE and its descendants have been exercised in a wide variety of analysis projects, so their performance has come under the scrutiny of individuals familiar with human operator performance. However, there has been no formal validation.

The SAMPLE architecture provides a general framework for constructing models of operators of complex systems, particularly in cases in which the operators are engaged in information processing and control tasks. SAMPLE draws heavily on modern control theory, which has enjoyed considerable success in the modeling of human control behavior. The belief net core of the situation assessor of later variants appears to have considerable potential for representing situation awareness. However, procedure development for SAMPLE models would appear to be quite labor-intensive since there seems to be no high-level procedure representation language.

Soar is a symbolic cognitive architecture that implements goal-oriented behavior as a search through a problem space and learns the results of its problem solving (Newell, 1990; see also Chapters 5 and 8). Complete discussions of Soar describe a hierarchy from an abstract knowledge level down to a hardware- or wetware-dependent technology level (Polk and Rosenbloom, 1994). However, the two middle levels—the problem-space level and the architecture level—are of primary