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How Do You Interpret The IQ Test Scores?

What is a good IQ score? What is a high IQ score? What is a low IQ score? These are common questions, particularly after someone finds out their score from an IQ test.

There are different types of IQ tests with each having their own scale. Hence two people have an IQ of 120 on different scales might not necessarily have the same intelligence level. However all tests and scales use 100 as the IQ of a person of average intelligence.

Since the average IQ is said to be 100, hence anything above 100 is considered as above average and below 100 as below average. IQ below 70 is taken as that of mentally challenged person. IQ above 140 is generally considered as that of a genius.

We already know that  IQ = Mental Age/Chronological Age X 100

Hence, the normal or average IQ score is 100, which means that the mental and chronological ages coincide.

It is true that modern intelligence tests now apply statistical methods to produce a score reflecting one's performance compared to the average performance of his peers. However, the normal or average IQ score will always remain 100.

Let us see why?

IQ Bell Curve

When we plot a sample of the population's IQ scores on an IQ against percentage chart, it is observed that the population distribution on the IQ scale forms a symmetrical bell-shaped curve. This is known as the "IQ Scale Bell-Curve". It reveals that on an IQ scale of 60 to 100, the number of people with those IQ scores increases and maximum people touch the 100 mark. Then, as the IQ scale increases from 100, the percentage of people with those scores starts decreasing in proportion to the start of the curve. A very small percentage of the population reaches an IQ of over 140.

From this chart, it can be thus concluded that a normal IQ score is generally considered to be around the 100 mark.

IQ scores are thus calculated on a scale of 0 to 200. A person scoring zero would be literally brain dead, while a score of 200 would mean that the person is probably the smartest person alive on the earth.

Since the first IQ test was developed by Alfred Binet and Theophile Simon and their IQ was classified on Simon-Binet scale which was later revised by Lewis Terman into Stanford-Binet scale, this is the most popularly used scale till date and all other modern scales also revolves around it.

According to Stanford-Binet scale, IQ is classified as following:

  • Over 140 - Genius or almost genius
  • 120 - 140 - Very superior intelligence
  • 110 - 119 - Superior intelligence
  • 90 - 109 - Average or normal intelligence
  • 80 - 89 - Dullness
  • 70 - 79 - Borderline deficiency in intelligence
  • Under 70 - Feeble-mindedness

Apart from the Stanford Binet-Scale, another scale popularly used is the Wechsler scale. Here, IQ is classified as:

  • Over 130 - Very superior
  • 120 - 129 - Superior
  • 110 - 119 - High average
  • 90 - 109 - Average
  • 80 - 89 - Low average
  • 70 - 79 - Borderline
  • Under 70 - Extremely low/intellectually deficient

Besides, these two scales which use a standard deviation of 15, another scale in popular use is the Cattell's scale which uses a standard deviation of 24. According to Cattell's scale, IQ is classified as:

  • Over 160 - Genius Level
  • 140 - 159 - Highly Intelligent
  • 120 - 139 - Above Average
  • 100 - 119 - Average
  • 90 - 99 - Below Average

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