However much we might disagree as to how close a correlation might be expected between the Binet tests and school level, independent of the relation to life-ages, or which is the better test, it is certain that they afford two different symptoms of mental deficiency. It becomes our immediate problem, therefore, to discover how the most information may be gained from a careful interpretation of the test of school level. If we had sufficient data, three sorts of checks might be formulated. 1. What amount of school retardation will give us the best estimate of mental deficiency among groups? 2. What amount of school retardation should put an individual's mentality in question so that he should be examined? 3. What amount of school success should put in question a Binet diagnosis? A. PRACTICAL USES OF THE SCHOOL TEST. (a) ESTIMATING THE FREQUENCY OF DEFICIENCY BY SCHOOL RETARDATION. We shall first take up the question of utilizing information about school retardation in estimating the frequency of mental deficiency among groups of delinquents. It is perfectly clear that retardation in school position is not always an indication of mental retardation. A child may be behind the position in school reached by the children of his age merely because he has not attended school so long as his companions. A census of school progress which we took in Minnesota indicates that in general a large part, perhaps half, of the retardation in school is to be thus explained even under compulsory attendance laws. Some allowance is also to be made for physical handicaps, such as defects of sight and hearing which are not corrected, illness which does not cause prolonged absence, frequent change of schools, bad home conditions, etc. Aside from absence, however, there can be no question that greater or less degrees of mental retardation is the main cause of retardation in school. Moreover a dull mind is often the reason for beginning school at an older age and for staying away from an unsuitable school environment as much as the law will permit. In any particular case, it is to be noted, however, that all of the excuses for backwardness in school are not likely to account for more than one or two years of lagging for other reasons than dullness.