This is why it's much harder to measure IQ after adulthood - particularly once your brain finishes cognitive development (which was traditionally thought to be at age 15 but more modern research is now suggesting age 25).
A 9 year old who can read at someone four years older (13 year old) is considered to be very intelligent and possibly placed in some accelerated learning programme whereas conversely a 13 year old with a reading age of a 9 year old would have most educators and parents seriously worried and possibly placed in some remedial learning assistance programme.
This comparison means nothing amongst adults though. A 19 year old who reads at the age of a 23 year old or a 23 year old who reads at the age of a 19 year old?? There's no real difference between the reading ages of a 19 and 23 year old.
QUOTE (thomas shaw)
I'm not at all sure where you got those formulas
Here's an excerpt from my notes on special education (March 2000)
Reference:
Howard, UNSW-------
IntelligenceThe number of children with special needs has increased partially because of advances in medicine. Children who would have otherwise died at an early age ('selected' out by nature, from a Darwinian viewpoint) are now living longer.
Gifted children exhibit symptoms such as reading and asking difficult questions at an early stage.
However, some children will not seem neither mentally deficient nor gifted until they begin school. This is because schooling is an unnatural environment. Hence, some children who may function normally at home will seem deficient at school. Often called "six-hour retardation" in some parts of the United States.
Most of these kids will leave school early and blend into society becoming fairly normal and productive members of society. However, this is becoming increasingly difficult as the number of unskill jobs decline as our society becomes more complex.
Many children will try to disguise their special needs. Schools are primarily designed to deal with average children and do not cope well with variation. Schools will try to homogenise students as much as possible. e.g.: streaming (organising classes within a grade by student intellect)
Consequently the slower students often get left behind whilst the brighter ones become bored.
People with a general deficit tend to be bad at doing everything and are considered to have special needs.
People with a general surplus tend to be good at everything and are considered to be gifted.
Texts on special education often do not cover the issue of intelligence very well.
Scientists have been studying intelligence for over a century. Intelligence is basically a function of the brain. The Ancient Greeks used to think intelligence came from the heart.
The brain doesn't function as a single unified thing. Rather, there is a whole set of largely independent units known as modules, consisting of several brain cells sitting together.
e.g.: modules that process visual information, audio information, social information, recognising faces etc.
For complex functions, several modules will loosely cooperate e.g.: music.
The brain modules have tacked onto each other throughout evolution... like a shack being renovated until it becomes a house.
Different modules develop at different rates. Some modules are functional at birth, whereas others develop throughout life through external stimulation.
Some modules work well in some people but not in others. This can be caused by genetics or brain damage.
The modular nature of the brain can be a cause of mental retardation. The modules form a very loose confederation, coined "the society of minds," by Marvin Minsky.
Most of our mental functioning is unconscious. The conscious mind is merely the tip of an iceberg. It is like the PR officer of a large corporation.
This is why people are sometimes unable to justify some of their actions.

However, there is a general intelligence.
Some brains seem to work faster and more efficiently than others. This equates with our perception of bright and dull people.
Our intelligence level determines our ability to think and work. Therefore it is a critical factor in school and life.
Intelligence also determines how far people will go in the education system. It determines how fast you can learn and what you can learn.
In all our endeavours, we all reach a point when something simply becomes too difficult to progress any further. There is a natural barrier.
Some people compensate by putting in lots of study.
IQ TestingThis is an imperfect, yet reasonable measure of intelligence. It also correlates reasonably well with grades and how well someone will perform in life.
However, not it is not the only factor to success. Other factors include motivation, connections, luck, personality and creativity.
Inate TalentSome argue that this does not exist and that ability is only dependent on motivation.
However, others argue that the above statement defies common sense.
What is intelligence?This is a very controversial question which dates back several thousand years.
Plato made a distinction between men of gold (the best), silver and bronze (average).
There is no universally agreed definition of intelligence.
In 1921, a survey was conducted where people were asked to define intelligence. There were over 13 definitions, including:
- ability to carry on abstract thinking
- capacity to carry on capacity
- proficiency in cognitive performance
- differential ability to solve problems
A survey in 1981 by Stermberg found that there were three major factors in intelligence:
1. Verbal ability (ie: someone is intelligent if they have good verbal ability)
2. Social competence
3. Practical ability (able to deal well with everyday problems)
The reason why there is so much confusion is because intelligence is a concept (like liberty, art and civilisation). Therefore people have different ideas and notions of what it is. Thus, although the same word is used, people attach different concepts to them. Therefore it is necessary to identify the different concepts.
These include:
a. Intelligence is property of behaviourThis argues that intelligence is a characteristic of behaviour, therefore an action is either intelligent or unintelligent.
b. Intelligence is a term for intellect in generalCognitive ability, special abilty etc. are all intelligence.
c. Spearman's g (g = general intelligence)Abilities only correlate to averages. Therefore, no matter how great one's intelligence is, everyone has their strengths and weaknesses due to the modular nature of the brain.
According to concepts a and b, one cannot argue that intelligence is inate (whereas concept c says it is).
IQ tests are used to identify the gifted, retarded, the learning disabled etc.
Although it is not an infallable means of testing intelligence, it is a generally good one.
In 1904, the education minister of France wanted to distinguish between lazy and mentally deficient children.
A psychologist by the name of Alfred Binet developed a test with thirty items (questions) in 1905. These questions included showing pictures of people and asking the children what emotions they were showing as well as pictures of certain objects.
This test was developed to determine a child's mental age (regardless of physical age). Binet said that the test was not a measure of intelligence, merely a diagnostic to discriminate between laziness and retardation.
Wilhelm Stern suggested:
(Mental Age / Physical Age) x 100 = Mental Quotient
Therefore a ten year child who is mentally seven has an MQ of 70 [because (7/10)x100 = 70, fairly low] whereas a six year old with the mental age of a twelve year old has an MQ of 200 [(12/6)x100=200, fairly high].
In 1917, an academic in California USA, developed the Stanford-Binet test.
This used Stern's formula, but was modified to become:
(Mental Age / Physical Age) x 100 = Intelligence Quotient (IQ)The use of the IQ test has spread far beyond the education system (industry etc). However, the test becomes less effective after the age of 16 or 17 (mental age doesn't change much after that). However, since physical age will continue to increase, the actual IQ will diminish with age.
Weschler developed a test in the 1940s known as the Weschler Adult Intelligence Scale (WAIS).
Weschler expressed IQ in terms of distribution as illustrated below:

...that'll do.