Most tests provide overall scores and individual subtest scores. This allows for an overall assessment of intelligence and your performance in each of the different areas. IQ tests are based on normal distribution bell curve. For this reason, they are only really designed and valid for certain IQ ranges. If you fall into the extremes of the scale they are highly unreliable. It is also interesting to know that your IQ score will not necessarily remain the same over your lifetime!
Here is a good overview video.
Origins of IQ tests
During the early 1900’s, the French Government wanted to find out which students were most likely to struggle in school. The French Government had just passed laws making school attendance compulsory. It was for this reason that it seemed a good idea to identify which children would need special assistance as early as possible. Binet, with the help of his colleague Theodore Simon, began to work on a question set that would let them assess a person’s ability to solve problems, remember facts and assess their attention span. This set of questions then formed the foundations for allowing them to predict likely success in school.
They quickly came to realize that certain children were able to answer advanced questions that older children could answer and vice versa. From these observations, Binet, suggested the concept of mental age. This was intended to be used a metric to determine the average intelligence per age group. Soon after Binet began to develop the first intelligence test. We call this the Binet-Simon scale today which has become the basis for intelligence tests. Despite developing the test Binet was not convinced that psychometric instruments could be used to measure a single, life-long, inborn level of intelligence.
Binet believed that intelligence was such a broad subject that giving a numerical value was insufficient. He believed that intelligence was influenced by a number of factors. He also believed that these changed over time and would only be comparable to children with similar backgrounds and experiences.
IQ test improvements
Standford University, namely psychologist Lewis Terman, took the Binet-Simon Scale and standardized it for American participants. The adapted test, The Stanford-Binet Intelligence Scale, became the standard intelligence test in the US in 1916. From this test, the term Intelligence Quotient was coined and it consisted of a single number. This number was meant to represent an individual’s performance from the tests results. The score was generated by taking the test maker’s mental age by their physical age and the multiplying the result by 100. So, by way of example, if a child acquired a mental age of 12 from the test, but they were 10 years old, their final IQ score would be 120.