We outline a measurement theory developed by integrating ideas about knowledge level analysis, production system models of transfer, additive conjoint measurement, and Rasch models of measurement. Productions are assumed to rq)resent situation-action elements of knowledge. The model views the performance of such a knowledge element as the combination of affordance properties associated with the element and ability properties associated with an individual. Under specified conditions, observed behavior can be used to separate and quantify variables measuring situation-action affordances and subject abilities. A specific version of this model is applied to data from four studies involving the CMU Lisp Tutor.