# How to get someone’s g

“Intelligence”, “IQ”, “*g*“, are terms that are often bandied around. The following may be helpful: the gist of how to get someone’s *g* score, which is often used as the measure of someone’s IQ (e.g. that’s the “IQ”/”intelligence” that’s referred to in the recentish BBC news article linking childhood intelligence and vegetarianism).

- Give many people a load of tests of ability.
- Zap everyone’s scores with PCA or factor analysis.
*g*is the first component and usually explains around 60% of the variance. Here’s a picture of*g*with some other components.- Use the component to calculate a score. For factor analysis there are many ways to do this, e.g. Thompson’s scores, Bartlett’s weighted least-squares. The gist is that for each person you compute a weighted sum of their scores where the weights are a function of how loaded the particular test score was on
*g*. - To get something resembling an IQ score, scale it has a mean of 100 and a s.d. of 15.

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Related:

General intelligence factor

http://en.wikipedia.org/wiki/G_factor

g, a Statistical Myth

http://www.cscs.umich.edu/~crshalizi/weblog/523.html