Historically, IQ was calculated using a formula comparing mental age to chronological age, but modern IQ tests use a different, statistically-based approach.
Historical Method: Mental Age vs. Chronological Age
The original method for calculating IQ involved dividing a person's mental age by their chronological age and multiplying by 100.
Formula:
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IQ = (Mental Age / Chronological Age) x 100
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Mental Age: A measure of a child's intellectual performance relative to other children of that age.
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Chronological Age: A person's actual age in years.
Example:
If a 10-year-old child performs on an intellectual test at the level of a 12-year-old, their IQ would be:
- IQ = (12 / 10) x 100 = 120
Limitations:
This method had significant limitations, particularly for adults. Mental age tends to plateau in adulthood, making the formula less meaningful.
Modern Method: Deviation IQ
Modern IQ tests, like the Wechsler Adult Intelligence Scale (WAIS) and the Stanford-Binet, use a "deviation IQ" approach. Instead of comparing mental age to chronological age, they compare an individual's performance to the performance of others in their age group.
Key Features:
- Standardization: IQ tests are standardized on a large, representative sample of the population.
- Normal Distribution: The scores are designed to follow a normal distribution (bell curve), with the average score set at 100.
- Standard Deviation: The standard deviation is typically set at 15. This means that approximately 68% of people score between 85 and 115.
How it Works:
- An individual takes the IQ test.
- Their raw score is compared to the scores of others in their age group who participated in the standardization sample.
- The raw score is then converted to a standardized score, reflecting their performance relative to the average.
Interpretation:
| IQ Score | Classification |
|---|---|
| 130+ | Very Superior |
| 120-129 | Superior |
| 110-119 | High Average |
| 90-109 | Average |
| 80-89 | Low Average |
| 70-79 | Borderline |
| Below 70 | Extremely Low (often associated with intellectual disability) |
Modern IQ calculation relies on statistically comparing your performance on a test to the average performance of people in your age group, resulting in a score that reflects your relative standing.
