Some problems connected with the process of designing a criterium-referenced test of the degree of development of the ability to write equations are described. Specific features of such diagnostical test are considered; and - on the basis of a psychological analysis - the mental operations constituting the ability to be tested (i. e. the test's criterium) are established. Relation between the manner of dealing with instructional problems and with the test's items in discussed with particular accent on individual differences between the school-children revealed by means of the test.