In a program of Individually Prescribed Instruction (IPI), where a student's progress through each level of a program of study is governed by his performance on a test dealing with individual behavioral objectives, there is considerable value in keeping the number of items on each test at a minimum. The specified test length for each objective must, however, be adequate to provide sufficient information regarding the student's degree of mastery of the behavioral objective being tested. Just what the minimum acceptable length will be depends on the manner in which test information is used to make decisions about individual students, the level of functioning required for defining mastery of an objective, the relative losses incurred in making false positive and false negative decisions, the background information available on the student and on the instructional process, and the premium on testing time within the instructional process. Some broad guidelines regarding test length of IPI posttests are included. A number of tables present data on the probability of the students achieving mastery level. (Author/MV)