He found a trend for all such tests to correlate positively with each other , which is called a positive manifold .
The psychologist Charles Spearman early in the 20th century carried out the first formal factor analysis of correlations between various test tasks .
psychologist found a trend for all such tests to correlate positively with each other , which is called a positive manifold .
The psychologist Charles Spearman early in the 20th century carried out the first formal factor analysis of correlations between various test tasks .