
American psychologist Lewis Terman (1877-1956), 1916 inventor the modern IQ scale (0-100); a Binet-Simon scale focused on testing the ‘average’ student, using William Stern's 1912 proposal that an individual's intelligence be measured as an MQ (test-age/actual-age). [1]

In IQs, Terman IQ is an IQ determined by the calculations and methods of American psychologist Lewis Terman, a psychology professor at Stanford University, who in 1916, building on the previous child development IQ work of Alfred Binet (1899), Theodore Simon (c.1890), and William Stern (1912), invented the modern version of the IQ scale, as explained in his book The Measurement of Intelligence, as follows:

IQ of 0 = no intelligence
IQ of 100 = average intelligence (of a normal child)

The name "Terman IQ" is synonymous with "ratio IQ", with the exception that the IQ value assigned to that person was determined by Terman himself, by analysis of that child's abilities.

200-range IQs
The first 200 range IQ was calculated by Terman in his 1917 article "The Intelligence Quotient of Francis Galton in Childhood", and assigned to Francis Galton, in which, in short, based on comments by Galton at age four:

“I am four years old and I can read any English book. I can say all the Latin substantives and adjectives and active verbs besides fifty-two lines of Latin poetry. I can cast up any sum in addition and can multiply by 2, 3, 4, 6, 8, 9, 10, 11. I can also say the pence table. I read French a little and know the clock.”

among other mental feats, in Galton's adolescence, Terman arrived at the result that Galton at age four was as smart as an average eight year old:

$IQ_{Galton_{age4}} \cong 100 \frac{8}{4} = 200 \,$

Thus, Terman concluded that via his age ratio IQ method, during the ages of about four to eight, Galton had a "intelligence quotient" of about 200. The central issue with this calculation is that it does not carry over into an adulthood IQ representation. Nevertheless, once someone is assigned a 200 IQ, people assume that this IQ value is representative of that person's intelligence, in retrospect, throughout their life. In other words, later when someone comes across the value the value of 190 assigned to Newton, by Catherine Cox (1926), people will reach the illogical conclusion that Galton was smarter than Newton, which makes no sense at all. To exemplify, Terman, using the above method, assigned Nicolas Copernicus, one of the greatest polymaths to have ever lived, the epitome of the "scientific revolution", with an IQ of 100 to 110:

$IQ_T \,$ =100-110 (Copernicus's IQ, as a child, according to Terman)

which was a value, no doubt based on Copernicus' childhood, which may have been average at age four to eight.

$IQ_C \,$ =160 (Copernicus' overall lifetime IQ, according to Catherine Cox, 1926)
$IQ_B \,$ =185 (Copernicus' overall lifetime IQ, according to Tony Buzan and Raymond Keene, 1994)
$IQ_{CB} \,$ =173 (Copericus' overall lifetime IQ, according to the mean of the Cox-Buzan estimates)
$IQ_? \,$ =200 (Copernicus's overall lifetime IQ, according to intuitive placement on the 2010 grouping of the 35-known EoHT 200+ range IQs)

In other words, Copernicus was far from average, but rather one of the greatest geniuses to have ever lived; subsequently, this example highlights the issues with making 200-range estimations, using the age ratio IQ method of Terman.


French psychologist Alfred Binet (1857-1911): in 1899 was appointed to the Commission for the Retarded, a repercussion of a new French law that mandated school for children ages six to fourteen, whose aim was to develop a test to differentiate between normal and abnormal children, so to be able assign each to different classrooms. [14]	French psychologist Theodore Simon (1872-1961): was an intern at the asylum in Perray-Vaucluse, studying abnormal children, during which time he began to work with Binet to develop a test that could measure intellectual development of children ages 3-12.	German psychologist William Stern (1871-1938): reviewed the work of Binet (and others), and developed the idea of expressing intelligence in the form of a single number, the "mental quotient" (1912) as one’s mental age divided by one’s chronological age: [15] $MQ = \frac{Age_{mental}}{Age_{actual}}\,$

IQ scale history
The IQ scale originated in the years to follow 1899 at which point French psychologist Alfred Binet (1857-1911) was assigned the task of facilitating the new French government mandate that all children ages six to fourteen go to school. Binet's aim was to develop a test for children ages 6-14, so as to be able to put each child in the appropriate classroom based on their test results. Binet's efforts eventually resulted in the now famous "Stanford-Binet Intelligence Test".

The point to remember here is that this test is geared for children aged fourteen our younger. Grandiose errors result when people extrapolate genius range IQs from this test, whereas in reality the test Stanford-Binet IQ test can only say that the child is at most at least as smart as fourteen year old.

Mental Quotient | MQ
In 1912, German psychologist William Stern (1871-1938) developed the formulaic idea that one could use childhood intelligence tests, such as Binet's, to put the test result into a formula, called a "mental quotient" (MQ), wherein the age the test was designed for, called the "mental age" (MA), would be divided by the age of the test taker, called the "chronological age" (CA), as follows:

$MQ = \frac{MA}{CA} \,$
or $MQ = \frac{Age_{mental}}{Age_{actual}}\,$

$MQ = \frac{MA}{CA} \,$	or	$MQ = \frac{Age_{mental}}{Age_{actual}}\,$

The central issue with this formula was that it didn't give whole number results but rather decimal place fractions values such as 0.89 or 1.22, etc., which didn't seem to make much of an intuitive ranking sense.

Intelligence Quotient | IQ
In 1916, to remedy the fraction issue of the Stern's mental quotient formula, Terman devised the idea that Stern's MQ should be multiplied by the value of 100, thus always giving whole number results (1, 2, 3, etc.) and thereafter be called an "Intelligence Quotient" (IQ):

$IQ = 100\frac{MA}{CA} \,$

Hence the idea of the "IQ" was born.

References
1. Terman, Lewis. (1916). The Measurement of Intelligence: an Explanation of and a Complete Guide for he Use of the Stanford Guide for the Use of the Stanford Revision and Extension of the Binet-Simon Intelligence Scale (I.Q., pg. 53, etc.). Houghton Mifflin Co.
2. Terman, Lewis. (1917). “The Intelligence Quotient of Francis Galton in Childhood,” American Journal of Psychology, 28: 209-15.

Further reading
● Terman, Lewis. (1930). Genetic Studies of Genius, Volume Three: The Promise of Youth; Follow-up Studies of a Thousand Gifted Children, by B. S. Burks and Others (Shelley IQ, pg. 366). Stanford University Press.