As representative of the first form of scale we may refer to the widely used Binet-Simon scale for the determination of mental age. Whatever we mean by intelligence, it is a characteristic which is essential to vocational activity. It is furthermore a characteristic which normally tends to increase in its degree or manifestation from infancy up to at least ten or twelve years of age. Beyond that point there are, to be sure, striking individual differences in that characteristic which we call intelligence, but beyond this point it does not seem so dependent on the physical age of the organism. Five-year-old children tend to be pretty much alike in intelligence. At least, the change from five years to seven years is commonly attended by very apparent growth in this respect, and a five-year-old is more like other five-year-olds in the things he can do than he is like seven-year-olds.
Experiment and observation show that the ages up to ten or twelve tend to indicate rather definite mental status, in the long run, although, to be sure, children of a given age vary considerably from one another. But beyond this point the age of an individual is not by any means an indication of the sort or degree of ability to be expected of him. The further we go beyond this point, the less significant becomes the mere statement of the individual's age. We may thus indicate the mental attainment of a child of less than twelve years by stating the average age of children who can do the things, know the facts, display the abilities that he can. This figure we will use to indicate his _mental_ age as distinguished from his _chronological_ or _physical_ or _actual age_. A record-blank which enumerates the tests comprising the Binet-Simon scale is given in the Appendix. Those who may be interested in using this or similar scales should familiarize themselves with some of the many books and manuals that have been written concerning them, the methods of using them, their characteristic results and their evaluation. These scales will be again considered in a later section, when we discuss the measures of general intelligence as they relate to vocational guidance and selection.
Other scales than the Binet-Simon series have been proposed, and this series has itself undergone modifications at the hands of later investigators--changes calculated to render it more reliable and adaptable. Much work is now being done in the attempt to develop scales or sets of tests which will reveal characteristic differences among people whose mentality has gone beyond the point which the juvenile scales reach.