The Intelligence Quotient- or IQ- is one of the most popular subjects in psychology. Yet despite us often using IQ as a shorthand for intelligence, and even using it to define others, misconceptions about IQ are often louder than explanations.

So how do IQ tests work, and what does an IQ score mean?

1) An IQ test does not directly measure your ability. It uses maths to estimate your ability in relation to other people.

If you want to find out how tall someone is, you grab a tape measure and hold it up to the person. A tape measure is a direct measurement; the number you read is the person’s height.

IQ is more abstract. In maths, the way a set of numbers or data is spread out is called a distribution. While there are hundreds of named distributions, many sets of numbers – such as people’s height, blood pressure readings, and body temperatures – follow the same bell-shaped pattern. This symetrical pattern, where the majority of results cluster around the centre, is named a normal distribution. (People talking about IQ often call it a Bell Curve as well).

IQ tests are designed so that their results, IQ scores, create a normal distribution. When an IQ test is developed or updated, it is given to a test group of thousands of people. In the same way that raw exam scores are converted into adjusted scores so that a fair balance of grades can be awarded, raw test scores are converted into adjusted IQ scores which fairly cover the entire curve.The test group’s mean (average) test score then becomes the centre point of the IQ scores, which is always set to 100. All future test-takers who get that mean raw score will be given an IQ score of 100.

Another important number for IQ tests is standard deviation (how far scores are spread out from the mean). In a normal distribution, 68% of the numbers are within one standard deviation of the mean; 26% are more than one one standard deviation above or below the mean, and only 4% are more than two standard deviations above or below. Most IQ tests set their standard deviation at 15 points. If somebody has an IQ result of 115, it doesn’t mean they got 15 more answers correct, but that they performed one standard deviation better than the average person.

So, rather than being direct like height, IQ score is an indirect measurement. It estimates someone’s position on that normal distribution curve of results by seeing what percentage of test-takers they scored more highly than. This is why High-IQ societies set their entry requirement as “an IQ in the top 2%/1%/0.1% of the population”, rather than a specific IQ score.

2) An IQ test is not one test with one result. It is a collection of tests for separate skills, which are then combined.

IQ tests contain 10-20 subtests that each measure a narrow area of ability, including visual pattern-matching, arithmetic, visualising 3-D shapes, and using words in context. People’s raw scores on each subtest are compared to normalised samples (data about typical scores for test groups of various demographics) then added together.

These smaller scores are averaged into two or three larger categories. The most popular IQ test in America, for example, is the Weschler Adult Intelligence Scale, which uses two categories. Verbal IQ covers all verbal and linguistic skills, while Performance IQ covers non-verbal skills such as shapes, space and visual patterns. Someone’s Verbal and Performance scores can usually be averaged together to give their Full-Scale IQ (an overall IQ result). If someone had a Verbal IQ score of 119 and a Performance IQ score of 111, their Full-Scale IQ score would be 115.

However, the important word in that sentence was usually. In the next post I’ll go through misconceptions about what individual IQ results mean and whether a high score on an IQ test can predict anything important about people.