When noting that all IQ tests correlated with each other, he found that there is one general factor of intelligence, which he called factor 'g'. That is the factor intelligence-test batteries measure. Though, this correlation wasn't high, Spearman explained low correlations stating that, in addition to the general intelligence factor, there were also specific factors, with a different specific factor associated with each test. American Louis Thurstone didn't agree with Spearman, saying that factor analysis should be based on simple structure. His method was to find a factor solution detecting factors that correlate highly with some tests, but don't correlate at all with the others.
Crystallized intelligence is based on knowledge and previously learned skills and abilities.
Achievement tests are often a good example of that. Fluid intelligence is seen in reasoning, memory and information processing. Crystallized intelligence reaches its peak later in life than does fluid intelligence, and it also declines much later. Combining the theories explained above, Carrol concluded that there is a hierarchical structure of intelligence. According to his model, there are three levels.