of students by getting evidence about a sample of his reactions. We do not collect all the written work the students have ever prepared in order to get some estimate of their writing ability. We recognize it is possible to judge the writing commonly to be expected from this student by examining a proper sample of his writing. Correspondingly, with reference to a student’s knowledge, we do not ask him all the questions about all the facts, principles, concepts and the like that may be involved in his education, but rather we choose a sample of these things to question him about and we infer from his reaction to this sample how he might react to the total set of items that might be involved in his knowledge. This holds for all types of human behavior, attitudes, interests, intellectual skills, appreciations, and the like. We assume that it is possible to infer the person’s characteristic performance by appraising his reaction in a simple of situations where this reaction is involved.