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An information-processing model is presented that describes how a person uses an interactive computer text-editing system to make modifications to a manuscript. It is demonstrated that the behavior of an expert user can be modeled by giving his goals, operators, methods, and selection rules for choosing among method alternatives. The paper assesses the predictions of such a model with respect to (1) predicting user behavior sequences, (2) predicting the time required to do particular modifications, and (3) determining the effect on accuracy of the detail with which the modeling is done (the model's “grain size”). Chronometric task protocols from several users are examined in some detail. Users' choices between alternative methods are predicted about 80% of the time by a few simple rules. Accuracy of the model is little affected by the detail of modeling. The manuscript-editing task is discussed as an example …