I. Q. stands for 'Intelligence Quotient' which is measure of a person's intelligence found by means of an intelligence test. Before marks gained in such a test can be useful as information about a person, they must be compared with some standard, or norm. It is not enough simply to know that a boy of thirteen has scored, say, ninety marks m a particular test. To know whether he is clever, average, or dull, his marks must be compared with the average achieved by boys of thirteen in that test. In 1906 the psychologist, Alfred Binet, devised the standard in relation to which intelligence has since been assessed. Binet was asked to find a method of selecting all children in the schools of Paris who should be taken out of ordinary classes and put in special classes for defectives. The problem brought home to him the need for a standard of intelligence, and he hit upon the very simple concept of' mental age'. First, he invented a variety of tests and put large numbers of children of different ages through them. He then found at what age each test was passed by the average child. Binet arranged the various tests in order of difficulty, and used them as a scale by which he could measure every individual. If, for example, a boy aged twelve could only do tests that were passed by the average boy of nine, Binet held that he was three years below average, and that he had a mental age of nine. The concept of mental age provided Binet, and through him, other psychologists with the required standard. It enabled him to state scores in intelligence tests m terms of norm. At first, it was usual to express the result of a test by the difference between the' mental' and the' chronological' age. Then the boy in the example given would be' three years retarded'. Soon, however, the' mental ratio' was introduced that is to say, the ratio of the mental age to the chronological age. Thus a boy of twelve with mental age of nine has a mental ratio of 0.75. The mental age was replaced by the intelligence quotient or' I. Q'. Clearly, since the mental age of the average child is equal to the chronological age, the average I. Q. is 100. To judge a child's standard, his marks in a test must be compared with marks gained by ______.