It's not one of a million metrics, it's the best possible summary of all the metrics: * There is very high correlation between different cognitive tests (peop

It's not one of a million metrics, it's the best possible summary of all the metrics: * There is very high correlation between different cognitive tests (people who are good at one test tend to be good at others) * If you perform factor anaylsis, you can compute a common factor that can be regarded as a summary variable for the correlations between all the different tests. * This is the g-factor, or the single number produced from IQ tests.