of investigating cognitive and perceptual processes. It has been applied quite effectively in studies ranging from an analysis of strategies used to take well-known intelligence and aptitude tests (e.g., Dillon, 1985; Just and Carpenter, 1992) to analyses of strategies used to process and comprehend sentences (Just and Carpenter, 1992) and even of how students interact with a computer-based cognitive tutor (Anderson and Gluck, 2001).

For many aspects of cognition that unfold over periods of time greater than a few seconds, methods such as the analysis of reaction-time data are less effective. The processes of cognition may be extremely complex, involving multiple mental operations, and they may be highly dependent on the contents of long-term memory. To study such situations, researchers often develop a detailed model of the knowledge structures and processing steps used to perform a particular task or solve some problem. Many such models are developed as formal computer models that embody a variety of assumptions about the nature of the cognitive architecture and the form in which knowledge is represented in the mind. It is common for these models to be written in the language of production systems containing explicit statements about what people know when they perform a task and the sequence of mental events that occur as the task is performed. Such models are not arbitrary because they are developed from observations and logical analysis of task demands. Most important, they are tested by examining the quality of the fit between results obtained from the simulation and actual human performance. The approach has been used in a wide variety of domains, from young children’s understanding of how balance scales work (Klahr and Siegler, 1978), to high schoolers’ learning of geometry (Anderson and Boyle, 1985), to adults’ performance on text editing (Singley and Anderson, 1988) and spatial navigation (Anderson, Kushmerick, and Lebiere, 1993), to the solution of intelligence test problems such as Raven’s Matrices (Carpenter et al., 1990).

Rich sources of information about the thinking process are offered by records of what people say as they go about solving a problem. These reports, referred to as concurrent verbal protocols, offer a window on the fleeting contents of working memory. An example is a person’s string of utterances while solving a mental arithmetic problem; these comments might refer to partial and temporary results or to the goals and subgoals the person generates during the episode. What a person says, however, should not be viewed as a reliable description of the underlying cognitive strategies being