The stratergy

If we have a set of test scores which are mutually positively correlated, we have argued that they may share something in common. This ‘something’ we have tentatively identified with g (although we have recognised the possibility that g might not be the only common influence). The question now is how to separate out what is common from what is peculiar to each test score. In mining terms we have a considerable quantity of ore which we know contains gold and we have to find a technique for separating the one from the other. Pursuing the metaphor, it is natural to speak of this process as ‘extracting’ g, though ‘exposing’ g would be more accurate in some respects. Factor analysis is the tool for carrying out the extraction. Lest this way of speaking raise false expectations, we should make clear at the outset that the metaphor is imperfect and that it may not be possible to extract g in anything like a pure form.

Factor analysis is not an elementary method and we shall make no attempt to give anything approaching a technical account. However, it is necessary to have some insight into how it works and what it can achieve. We shall adopt a threefold approach, each giving a different angle, with the intention that each will complement the others.