The calculation of mental age is really simpler than our verbal illustrations make it appear. After the operation has been performed twenty or thirty times, it can be done in less than a half-minute without danger of error. THE USE OF THE INTELLIGENCE QUOTIENT. As elsewhere explained, the mental age alone does not tell us what we want to know about a child's intelligence status. The significance of a given number of years of retardation or acceleration depends upon the age of the child. A 3-year-old child who is retarded one year is ordinarily feeble-minded; a 10-year-old retarded one year is only a little below normal. The child who at 3 years of age is retarded one year will probably be retarded two years at the age of 6, three years at the age of 9, and four years at the age of 12. What we want to know, therefore, is the ratio existing between mental age and real age. This is the intelligence quotient, or I Q. To find it we simply divide mental age (expressed in years and months) by real age (also expressed in years and months). The process is easier if we express each age in terms of months alone before dividing. The division can, of course, be performed almost instantaneously and with much less danger of error by the use of a slide rule or a division table. One who has to calculate many intelligence quotients should by all means use some kind of mechanical help. HOW TO FIND THE I Q OF ADULT SUBJECTS. Native intelligence, in so far as it can be measured by tests now available, appears to improve but little after the age of 15 or 16 years. It follows that in calculating the I Q of an adult subject, it will be necessary to disregard the years he has lived beyond the point where intelligence attains its final development. Although the location of this point is not exactly known, it will be sufficiently accurate for our purpose to assume its location at 16 years. Accordingly, any person over 16 years of age, however old, is for purposes of calculating I Q considered to be just 16 years old. If a youth of 18 and a man of 60 years both have a mental age of 12 years, the I Q in each case is 12 ÷ 16, or .75. The significance of various values of the I Q is set forth elsewhere.[44] Here it need only be repeated that 100 I Q means exactly average intelligence; that nearly all who are below 70 or 75 I Q are feeble-minded; and that the child of 125 I Q is about as much above the average as the high-grade feeble-minded individual is below the average. For ordinary purposes all who fall between 95 and 105 I Q may be considered as average in intelligence. [44] See Chapter VI.