The two factors of intelligence

Spearman concluded that two factors of intelligence existed:

1. A single general factor, the g factor (correlations)

and

2. Numerous specific factors, s1, s2, ... (differences across test scores).

The blue ovals represent individual mental tests,
areas shaded blue are due to variance of s,
and the areas shaded purple are due to the variance of g.

The IQ score was determined to be an accurate measure of mental development and of general cognitive functioning.

As such, IQ would indicate the average of the specific factors canceling each other out~ highlighting the g-factor.