The measurement of intelligence

The Binet scale is made up of an extended series of tests in the nature of “stunts,” or problems, success in which demands the exercise of intelligence. As left by Binet, the scale consists of 54 tests, so graded in difficulty that the easiest lie well within the range of normal 3-year-old children, while the hardest tax the intelligence of the average adult. The problems are designed primarily to test native intelligence, not school knowledge or home training. They try to answer the question “How intelligent is this child?” How much the child has learned is of significance only in so far as it throws light on his ability to learn more.

Binet fully appreciated the fact that intelligence is not homogeneous, that it has many aspects, and that no one kind of test will display it adequately. He therefore assembled for his intelligence scale tests of many different types, some of them designed to display differences of memory, others differences in power to reason, ability to compare, power of comprehension, time orientation, facility in the use of number concepts, power to combine ideas into a meaningful whole, the maturity of apperception, wealth of ideas, knowledge of common objects, etc.

The following is the list of tests as arranged by Binet in 1911, shortly before his untimely death:—

By means of the Binet tests we can judge the intelligence of a given individual by comparison with standards of intellectual performance for normal children of different ages. In order to make the comparison it is only necessary to begin the examination of the subject at a point in the scale where all the tests are passed successfully, and to continue up the scale until no more successes are possible. Then we compare our subject’s performances with the standard for normal children of the same age, and note the amount of acceleration or retardation.

Psychologists had experimented with intelligence tests for at least twenty years before the Binet scale made its appearance. The question naturally suggests itself why Binet should have been successful in a field where previous efforts had been for the most part futile. The answer to this question is found in three essential differences between Binet’s method and those formerly employed.

Binet was the first to utilize the idea of age standards, or norms, in the measurement of intelligence. It will be understood, of course, that Binet did not set out to invent tests of 10-year intelligence, 6-year intelligence, etc. Instead, as already explained, he began with a series of tests ranging from very easy to very difficult, and by trying these tests on children of different ages and noting the percentages of successes in the various years, he was able to locate them (approximately) in the years where they belonged.

The Binet method enables us to characterize the intelligence of a child in a far more definite way than had hitherto been possible. Current descriptive terms like “bright,” “moderately bright,” “dull,” “very dull,” “feeble-minded,” etc., have had no universally accepted meaning. A child who is designated by one person as “moderately bright” may be called “very bright” by another person. The degree of intelligence which one calls “moderate dullness,” another may call “extreme dullness,” etc. But every one knows what is meant by the term 8-year mentality, 4-year mentality, etc., even if he is not able to define these grades of intelligence in psychological terms; and by ascertaining experimentally what intellectual tasks children of different ages can perform, we are, of course, able to make our age standards as definite as we please.

In the second place, the Binet tests differ from most of the earlier attempts in that they are designed to test the higher and more complex mental processes, instead of the simpler and more elementary ones. Hence they set problems for the reasoning powers and ingenuity, provoke judgments about abstract matters, etc., instead of attempting to measure sensory discrimination, mere retentiveness, rapidity of reaction, and the like. Psychologists had generally considered the higher processes too complex to be measured directly, and accordingly sought to get at them indirectly by correlating supposed intelligence with simpler processes which could readily be measured, such as reaction time, rapidity of tapping, discrimination of tones and colors, etc. While they were disputing over their contradictory findings in this line of exploration, Binet went directly to the point and succeeded where they had failed.

It is now generally admitted by psychologists that higher intelligence is little concerned in such elementary processes as those mentioned above. Many of the animals have keen sensory discrimination. Feeble-minded children, unless of very low grade, do not differ very markedly from normal children in sensitivity of the skin, visual acuity, simple reaction time, type of imagery, etc. But in power of comprehension, abstraction, and ability to direct thought, in the nature of the associative processes, in amount of information possessed, and in spontaneity of attention, they differ enormously.

The assumption that it is easier to measure a part, or one aspect, of intelligence than all of it, is fallacious in that the parts are not separate parts and cannot be separated by any refinement of experiment. They are interwoven and intertwined. Each ramifies everywhere and appears in all other functions. The analogy of the stones of the tower does not really apply. Memory, for example, cannot be tested separately from attention, or sense-discrimination separately from the associative processes. After many vain attempts to disentangle the various intellective functions, Binet decided to test their combined functional capacity without any pretense of measuring the exact contribution of each to the total product. It is hardly too much to say that intelligence tests have been successful just to the extent to which they have been guided by this aim.

In devising tests of intelligence it is, of course, necessary to be guided by some assumption, or assumptions, regarding the nature of intelligence. To adopt any other course is to depend for success upon happy chance.

However, it is impossible to arrive at a final definition of intelligence on the basis of a-priori considerations alone. To demand, as critics of the Binet method have sometimes done, that one who would measure intelligence should first present a complete definition of it, is quite unreasonable. As Stern points out, electrical currents were measured long before their nature was well understood. Similar illustrations could be drawn from the processes involved in chemistry physiology, and other sciences. In the case of intelligence it may be truthfully said that no adequate definition can possibly be framed which is not based primarily on the symptoms empirically brought to light by the test method. The best that can be done in advance of such data is to make tentative assumptions as to the probable nature of intelligence, and then to subject these assumptions to tests which will show their correctness or incorrectness. New hypotheses can then be framed for further trial, and thus gradually we shall be led to a conception of intelligence which will be meaningful and in harmony with all the ascertainable facts.

It is interesting to compare Binet’s conception of intelligence with the definitions which have been offered by other psychologists. According to Ebbinghaus, for example, the essence of intelligence lies in comprehending together in a unitary, meaningful whole, impressions and associations which are more or less independent, heterogeneous, or even partly contradictory. “Intellectual ability consists in the elaboration of a whole into its worth and meaning by means of many-sided combination, correction, and completion of numerous kindred associations.... It is a combination activity.”

Meumann offers a twofold definition. From the psychological point of view, intelligence is the power of independent and creative elaboration of new products out of the material given by memory and the senses. From the practical point of view, it involves the ability to avoid errors, to surmount difficulties, and to adjust to environment.

Stern defines intelligence as “the general capacity of an individual consciously to adjust his thinking to new requirements: it is general adaptability to new problems and conditions of life.”

The above definitions are only to a slight extent contradictory or inharmonious. They differ mainly in point of view or in the location of the emphasis. Each expresses a part of the truth, and none all of it. It will be evident that the conception of Binet is broad enough to include the most important elements in each of the other definitions quoted.

It was in the assembling of the tests that the most serious faults of the scale had their origin. Further investigation has shown that a great many of the tests were misplaced as much as one year, and several of them two years. On the whole, the scale as Binet left it was decidedly too easy in the lower ranges, and too difficult in the upper. As a result, the average child of 5 years was caused to test at not far from 6 years, the average child of 12 years not far from 11. In the Stanford revision an effort has been made to correct this fault, along with certain other generally recognized imperfections.

The Binet tests have often been criticized for their unfitness to perform certain services which in reality they were never meant to render. This is unfair. We cannot make a just evaluation of the scale without bearing in mind its avowed limitations.

It should, nevertheless, be pointed out that defects of intelligence, in a large majority of cases, also involve disturbances of the emotional and volitional functions. We do not expect to find perfectly normal emotions or will power of average strength coupled with marked intellectual deficiency, and as a matter of fact such a combination is rare indeed. In the course of an examination with the Binet tests, the experienced clinical psychologist is able to gain considerable insight into the subject’s emotional and volitional equipment, even though the method was designed primarily for another purpose.

Finally, in arriving at an estimate of a subject’s grade of intelligence and his susceptibility to training, it would be a mistake to ignore the data obtainable from other sources. No competent psychologist, however ardent a supporter of the Binet method he might be, would recommend such a policy. Those who accept the method as all-sufficient are as much in error as those who consider it as no more important than any one of a dozen other approaches. Standardized tests have already become and will remain by far the most reliable single method for grading intelligence, but the results they furnish will always need to be interpreted in the light of supplementary information regarding the subject’s personal history, including medical record, accidents, play habits, industrial efficiency, social and moral traits, school success, home environment, etc. Without question, however, the improved Binet tests will contribute more than all other data combined to the end of enabling us to forecast a child’s possibilities of future improvement, and this is the information which will aid most in the proper direction of his education.

Tests of 400 of the 1700 normal children had been made by Childs and Terman in 1910–11, and of 300 children by Trost, Waddle, and Terman in 1911–12. For various reasons, however, the results of these tests did not furnish satisfactory data for a thoroughgoing revision of the scale. Accordingly a new investigation was undertaken, somewhat more extensive than the others, and more carefully planned. Its main features may be described as follows:—

1. The first step was to assemble as nearly as possible all the results which had been secured for each test of the scale by all the workers of all countries. The result was a large sheet of tabulated data for each individual test, including percentages passing the test at various ages, conditions under which the results were secured, method of procedure, etc. After a comparative study of these data, and in the light of results we had ourselves secured, a provisional arrangement of the tests was prepared for try-out.

2. In addition to the tests of the original Binet scale, 40 additional tests were included for try-out. This, it was expected, would make possible the elimination of some of the least satisfactory tests, and at the same time permit the addition of enough new ones to give at least six tests, instead of five, for each age group.

4. The children’s responses were, for the most part, recorded verbatim. This made it possible to re-score the records according to any desired standard, and thus to fit a test more perfectly to the age level assigned it.

5. Much attention was given to securing uniformity of procedure. A half-year was devoted to training the examiners and another half-year to the supervision of the testing. In the further interests of uniformity all the records were scored by one person (the writer).

As was to be expected, the first draft of the revision did not prove satisfactory. The scale was still too hard at some points, and too easy at others. In fact, three successive revisions were necessary, involving three separate scorings of the data and as many tabulations of the mental ages, before the desired degree of accuracy was secured. As finally revised, the scale gives a median intelligence quotient closely approximating 100 for the unselected children of each age from 4 to 14.

Since our school children who were above 14 years and still in the grades were retarded left-overs, it was necessary to base the revision above this level on the tests of adults. These included 30 business men and 150 “migrating” unemployed men tested by Mr. H. E. Knollin, 150 adolescent delinquents tested by Mr. J. Harold Williams, and 50 high-school students tested by the writer.

The extension of the scale in the upper range is such that ordinarily intelligent adults, little educated, test up to what is called the “average adult” level. Adults whose intelligence is known from other sources to be superior are found to test well up toward the “superior adult” level, and this holds whether the subjects in question are well educated or practically unschooled. The almost entirely unschooled business men, in fact, tested fully as well as high-school juniors and seniors.

The following method was employed for determining the validity of a test. The children of each age level were divided into three groups according to intelligence quotient, those testing below 90, those between 90 and 109, and those with an intelligence quotient of 110 or above. The percentages of passes on each individual test at or near that age level were then ascertained separately for these three groups. If a test fails to show a decidedly higher proportion of passes in the superior I Q group than in the inferior I Q group, it cannot be regarded as a satisfactory test of intelligence. On the other hand, a test which satisfies this criterion must be accepted as valid or the entire scale must be rejected. Henceforth it stands or falls with the scale as a whole.

A comparison of the above list with either the Binet 1908 or 1911 series will reveal many changes. On the whole, it differs somewhat more from the Binet 1911 scale than from that of 1908. Thus, of the 49 tests below the “adult” group in the 1911 scale, 2 are eliminated and 29 are relocated. Of these, 25 are moved downward and 4 upward. The shifts are as follows:—