This paper documents the achievement monitoring component of a three-year study on the acquisition of addition-subtraction problem-solving skills by young children. A set of performance objectives contained in or ancillary to ten instructional units on sentence-writing for verbal problems and algorithms specified test content. Tests measuring group progress toward these objectives were administered after each unit. Data for the tests given after the two units covered in the fall semester of grade 3 are presented in this paper. The scores for each objective, developed using matrix sampling procedures, indicated mastery of addition and subtraction without regrouping, mastery of the addition algorithm for numbers 0-99 and near-mastery of the subtraction algorithm for numbers 0-99. These results reflected the instructional emphasis. There was marked improvement on several of the problem-solving 0-99 and sentence-writing 0-99 objectives not previously mastered, though only one of these objectives was actually mastered. With the exception of the subt-join-addend sentence-writing 0-20 objective, progress (or maintenance) on the objectives associated with the numbers 0-20 was excellent. (Author)