The science of mental measurement is rapidly progressing to more exact usage. The concept of “mental age” when applied to persons who vary in birthday age is in some respects misleading, and in other respects quite inapplicable (as with superior adults). General intelligence is at present usually scored in terms of points achieved, percentile attained in total distribution, or of mental ratio. The most reliable scales now available for the measurement of general intelligence in school children, score in terms of mental age and intelligence quotient (IQ). This measure (IQ) signifies the ratio borne by the intellectual level attained by a given child in tests, to the level attained by the typical child of his birthday age. For instance, a child 9 years 6 months old has an IQ of 100, if his score in tests equals that made by the average child of 9 years 6 months. If he is inferior to the average child of his age, the amount of such inferiority will be expressed by a ratio less than 100. Thus, if his performance equals only that of the average child of 5 years 2 months, his IQ will be 62 months ÷ 114 months, or 54 (dropping fractions less than .5). On the other hand, if he is superior to the average, attaining, let us say, the performance of the average child of 14 years 0 months, his IQ will be 168 months ÷ 114 months, or 147. An IQ of 100 may thus be thought of as “par” in general intelligence for a school child, while anything less may be thought of as “below par” to the extent indicated; and anything greater than 100 may be thought of as “above par.” The IQ shows the point of focus, for amounts of performance in a variety of mental functions. It derives its value for educational procedure from the positive correlation, which has been demonstrated to exist among performances in mental operations. Scales at present available will measure general intelligence, in terms of IQ, about as low as IQ 10, and about as high as IQ 190, at certain periods of development. No doubt human intelligence ranges somewhat below and above these limits, but adequate methods of establishing the two extremes have not yet been devised. It is by no means usually realized that the range of individual differences in general ability is so wide that it is extremely difficult to invent methods of discovering its full extent. However, for practical purposes, available scales are adequate to cover the range for young school children, because intelligences that fall below IQ 25 or above IQ 175 are so rare as to be dealt with very seldom. Within the limitations named, the general intelligence of school children can now be determined by a competent examiner, with a very small margin of error. The average error made by such an examiner will not exceed ± 5 IQ.