Question
Why do IQ scores differ between tests?
Short answer
IQ scores differ between tests for four main reasons: different scales (a standard deviation of 15, 16 or 24), different norm samples, different kinds of items, and measurement error of a few points on any test. The same ability can print as 130 on one test and about 148 on another.
Want your own number? The test is 25 visual puzzles, about 12 minutes, no sign-up.
1. Different scales
Modern tests use a standard deviation of 15 (Wikipedia: Intelligence quotient). Older Stanford-Binet forms used 16, and the Cattell Culture Fair III scale uses 24 (Wikipedia: IQ classification). The same percentile prints as a different number on each:
| SD 15 | SD 16 | SD 24 |
|---|---|---|
| 85 | 84 | 76 |
| 115 | 116 | 124 |
| 120 | 121 | 132 |
| 130 | 132 | 148 |
| 140 | 143 | 164 |
Each row is the same position on the curve. Mensa UK, for example, quotes 148 on Cattell III B and 132 on the Culture Fair test as the same 98th percentile.
Try one · Quantitative Reasoning
Which number comes next?
6, 7, 11, 18, 28, 41, ?
Tap the option you think is right, or show the answer.
Show the answer
The next number is 57 (option 6).
How to solve it
- Find the gaps between neighbors: 1, 4, 7, 10, 13.
- The gaps between the numbers grow by 3 each time.
- So the next gap is 16, and 41 + 16 = 57.
That was 1 puzzle. The full test has 25.
2. Different norm samples
A score compares you with a reference group. Tests normed on different groups, or in different decades, place the average in slightly different places. That is one reason tests are renormed; see why IQ tests are renormed.
3. Different items
A full clinical battery samples many abilities. A nonverbal test such as a matrix test samples mostly reasoning with shapes. If you are stronger in one area than another, tests that weight those areas differently will give you different numbers.
4. Measurement error and practice
Every score has error; clinical manuals often report about plus or minus 5 points at 95% (Wechsler, 2008). Taking a second test also tends to raise scores by about a quarter of a standard deviation (Hausknecht et al., 2007). Two honest tests a few points apart may be telling you the same thing.
To compare across scales, use the percentile to IQ converter. For the major tests side by side, read WAIS, Stanford-Binet, Raven's and Mensa tests compared.
Sources
- IQ classification. Wikipedia. en.wikipedia.org/wiki/IQ_classification
- Intelligence quotient. Wikipedia. en.wikipedia.org/wiki/Intelligence_quotient
- Wechsler, D. (2008). WAIS-IV Technical and Interpretive Manual. Pearson.
- Hausknecht, J. P., Halpert, J. A., Di Paolo, N. T., & Moriarty Gerrard, M. O. (2007). Journal of Applied Psychology, 92(2), 373-385. doi.org/10.1037/0021-9010.92.2.373