Manifest and latent variables

We are now ready to formalise some of the ideas that have been illustrated in the previous two chapters in order to lay the groundwork for our later exposition of factor analysis. We begin with the most fundamental distinction of all. It is what distinguishes g from IQ, but it goes much wider. It concerns the difference between what, in technical language, are called manifest variables and latent variables. A variable is any quantity which varies from one member of a population to another – height and hair colour in the case of human populations, for example. A variable is manifest if it is possible to observe it and to record its value by counting or by using a measuring instrument like a ruler, clock or weighing machine. Thus, any variable whose magnitude can be observed and expressed in units of number, length, time or weight is a manifest variable. A great many variables which appear in social discourse are of this kind. The idea can be extended to cover anything calculated from a set of manifest variables, like an average, for example. In that sense IQ is a manifest variable because, as we have seen, it is something calculated from manifest test scores and is, therefore, itself, observable.

Many of the most important variables arising in the social sphere cannot be directly observed. In some cases this is because we do not have access to them.