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Methods of Measuring Intelligence: Interpreting IQ Scores & Score Range

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  • 0:05 What is IQ?
  • 1:44 The IQ Equation
  • 5:42 Interpreting IQ Scores
  • 8:22 IQ in School Today
  • 9:01 Lesson Summary
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Lesson Transcript
Instructor: Wind Goodfriend
You've probably heard of 'IQ,' but do you really know what it means? This lesson covers the history and theory behind the famous idea of an intelligence quotient (including the inventor of IQ, Alfred Binet), and will help you learn how to understand and interpret IQ scores.

What is IQ?

Imagine you're a ten-year-old child who lives on a farm in France during the turn of the century, around the year 1904. You're having trouble in school, learning how to spell all those French words and how to read books like Candide. But why are you struggling? Is it because you are low in intelligence? Or is it simply because you are from a poor family who doesn't have the same resources as a rich family? Maybe you are very intelligent, but your problems in school are due to pragmatic issues such as a lack of books in your house.

A man named Alfred Binet lived in France during this period and noticed that many children in school were struggling, but he didn't think it was due to lack of intelligence. So Binet made a very important distinction between two different things that might lead to poor school performance. He said that some children will do badly because they simply aren't intelligent, so there's not much that can be done to directly help them improve their grades. However, he said that other children have plenty of learning ability and potential to do well, but they struggle simply because of life circumstances. He said that these children, who have a high learning ability, should be distinguished from children who do badly in school because of low intelligence.

Alfred Binet made a distinction between high potential and low intelligence children
Alfred Binet Distinction

This distinction is what paved the way for Binet's new idea, which he called the intelligence quotient, or IQ for short. Everyone has an individual intelligence quotient, which is a number indicating your inherent ability to learn instead of simply the knowledge you already have. If you have a high IQ, it means that you have the intelligence required to do well in school because you have the ability or potential to succeed.

The IQ Equation

So how did Binet suggest that we measure someone's ability to learn or potential to succeed? Binet said that the way we can do this is to compare two things. First, we just need to know how old a child is, which he called a person's chronological age. What's your chronological age? Right now I'm 36, so that's my chronological age.

But Binet also said that we need to know your mental age, which is your cognitive abilities relative to what others can do of different ages. In other words, what do you think an average five year old can do? Can they read? Can they do math? What about the average ten year old? Or how about someone who is 40 years old? Your mental age is what you are capable of doing, scaled to what the average people of different ages can do. In other words, if you're capable of doing what an average 25-year old can do, then your mental age is 25. If you can do what an average 10-year old can do, but not anything more advanced than that, then your mental age is only 10.

How did Binet figure out what an average kid of each different age could do? He simply did research using a bunch of kids at each age to directly measure their cognitive abilities. Instead of measuring academic knowledge like, 'Who wrote the play Romeo and Juliet?', he measured ability to solve problems and think creatively, such as noticing patterns or remembering images. After testing lots of kids of all different ages, he could then give you, personally, the same test and compare your scores to what kids of each different age could do. If your scores matched what the 10-year olds were doing, then we know your mental age is 10 years old.

So now that we have these two scores - chronological age and mental age - we want to compare them. If you are right on track, then your two scores will perfectly match. If you are advanced, or very intelligent, according to Binet, then your mental age will be bigger than your chronological age. Say, if you are 20 years old right now, but you can do what an average 30-year old can do, then you're 10 years ahead of schedule, which Binet would say makes you very intelligent. The opposite could also be true; maybe you're 20 years old, but can only do what the average 10-year old can do. Binet would say that you are cognitively delayed and therefore not intelligent based on IQ.

To get a bit more technical, Binet created an equation so that we could all have a quick, simple number that indicates where we stand in comparing these two scores. The equation looks like this: Mental Age / Chronological Age x 100 = IQ. We can make a ratio of the two ages, with mental age on top, and the only reason we multiply by 100 is to get a nice, round number instead of a fraction. Let's go through a couple of examples of the simple math here.

If your mental age and chronological age are a perfect match, you might get an equation like this: 20 mental years old / 20 actual years old x 100 = 100. You can see that an IQ score of 100 will always mean that a person is exactly where the average person would be, so an IQ of 100 will always mean average intelligence for IQ. If a person is mentally advanced, they might get a score like this: 30 mental years old / 20 actual years old x 100 = 150. An IQ score of 150 would be very impressive! Just as an example, the rumor is that Albert Einstein had an IQ of 160! Any IQ higher than 100 means that the person is a little bit more intelligent than average. Again, the opposite can also be true. Let's say we have a person who is a little bit developmentally delayed; he or she might have an equation like this: 10 mental years old / 20 actual years old x 100 = 50. An IQ score less than 100 will always mean that a person's mental abilities are not as good as the average person of his or her age group.

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