Spearman concluded that performance on any test of mental ability could be explained by several cognitive factors organized hierarchically as one general factor and several subordinate specific factors (Spearman, 1904, 1927). The general factor accounted for the significant amount of inter-correlation between all ability tests for any one individual while the specific factor explained the variation that was unique to each test. While there has been much debate in the literature as to what the specific factors are, with many different combinations of number and type of ability, three abilities -- verbal, quantitative and spatial--consistently emerge as playing a dominant role in cognition (Kyllonen, 1996).