IQ stands for Intelligence Quotient, which is a 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 in 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. He invented a variety of tests and put large numbers of children of different ages through them. He found at what age each test was passed by the average child. For instance, he found that the average child of seven could count backwards from 20 to 1 and the average child of three could repeat the sentence: We are going to have a good time in the country. Binet arranged the various tests in order of difficulty, and used them as a scale against 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 has a mental age of nine. The concept of mental age provided Binet, and through him, other psychologists, with the required standard, which enables him to state scores in intelligence tests in terms of a 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 a mental age of nine has a mental ratio of 0.75. The mental age was replaced by the 'intelligence quotient' or 'ID'. The IQ is the mental ratio multiplied by 100. For example, a boy of twelve with a mental age of nine has an IQ of 75. Clearly, since the mental age of average child is equal to the chronological age, the average IQ is 100. Which of the following is not mentioned in relation to IQ?______