The psychological study of human intelligence is primarily the concern of psychometricians, or differential psychologists, who assume that intelligence exists as a set of quantifiable dimensions along which people can be measured. Typically, the psychometrician studies individual differences by administering test batteries that have demonstrated reliability to large numbers of subjects under controlled conditions. Scores from these tests are then analyzed by factor analysis or multidimensional scaling to extract underlying factors or dimensions responsible for patterns of correlations. Individual differences are usually interpreted in terms of factors that differentiate both the tests and the subjects. The models advanced to account for intelligence are expressed as factor structures. There are numerous expressions of these, ranging from those that emphasize a single general ability factor (G), to those that emphasize a large number of specific abilities.