generated through simulation of the human interacting with the task environment in simulated real time, in which the processors run independently and in parallel. Each model includes a process that represents the task environment and generates stimuli and collects the responses and their simulated times over a large number of trials. To represent human variability, the processor time parameters can be varied stochastically about their mean values with a regime that produces a coefficient of variation for simple reaction time of about 20 percent, a typical empirical value.

EPIC is currently available for Macintosh and various UNIX platforms. It is written in Common Lisp and therefore easily portable to other platforms. A graphical environment is provided on the Macintosh platform via Macintosh Common Lisp. An interface between EPIC's peripherals and Soar's cognitive processor is available on the UNIX platforms.

EPIC is a relatively new architecture that has not yet been the object of the development time and effort devoted to other architectures, such as ACT-R, Soar, and Micro Saint. Therefore, its support environment is not specific to EPIC, but depends on a conventional Lisp programming environment. The EPIC user can rely on several sources of documentation. In addition to publications describing the architecture and research results, Kieras and Meyer (1996) describe the various components of EPIC and their interaction. EPIC's source code, written in Lisp, is heavily annotated and relatively accessible.

Documentation and source code relevant to EPIC can be accessed at <ftp://ftp.eecs.umich.edu/people/kieras/>.

As mentioned earlier, EPIC is a relatively new integrative architecture, so it has generated fewer models than older architectures such as ACT-R and Soar. However, there has been rigorous evaluation of the EPIC models against human data. Currently, models that match human data both qualitatively and quantitatively exist for the psychological refractory period (PRP) (Meyer and Kieras, 1997a, 1997b), a dual tracking/stimulus-response task (Kieras and Meyer, 1997), a tracking/decision making task (Kieras and Meyer, 1997), verbal working-memory tasks (Kieras et al., 1998), computer interface menu search (Hornoff and Kieras, 1997), and a telephone operator call-completion task (Kieras et al., 1997).