“Intelligence”, “IQ”, “*g*” (due to Spearman), are terms that are bandied around.

The following may be helpful: the gist of how to calculate someone’s *g* score, which is often used as the measure of someone’s “intelligence”.

For example, that’s the “IQ”/”intelligence” referred to in the recentish BBC article on research linking childhood intelligence and adult vegetarianism (clever children grow into clever vegetarian adults).

- Give hundreds or thousands of people a dozen tests of ability.
- Zap everyone’s scores with PCA or factor analysis.
*g*is the first component and usually explains around half the variance. Here’s an example genre of analysis of*g*with other facets to psychometric intelligence.- 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 so it has a mean of 100 and an SD of 15.
- Talk about it as if it were a substantive psychological construct, rather than a statistical artefact 😉

What is this mysterious *g* thing?