The development and refinement of the measures of student achievement in reading and mathematics for the Beginning Teacher Evaluation Study are described. The concept of reactivity to instruction is introduced: the tests used to evaluate instructional processes must be sensitive indicators of classroom learning overtime. Data collection activities in second and fifth grade reading and mathematics classes are described. The analysis and evaluation of the reactivity of test items to instruction are summarized. General student ability and the amount of time devoted to instruction are also considered. Several reading test items are included to illustrate the methodology used. (GDC)