Construct, Vehicles, and Measurements
As a construct, the g factor can be represented with varying degrees of convenience, efficiency, and validity by a wide variety of vehicles (psychometric tests, laboratory techniques, physiological indices) that yield measurements that have different scale properties. These three key concepts are related to one another, but do not all represent one and the same thing. It is important to recognize the distinctions between them when considering the nature of empirically observed changes in objective mental measurements. These may be spontaneous changes in test scores within an individual, or a secular trend in the mean of a population, or score gains induced by training or other interventions.
The critical question, then, is the locus of the change. Does it represent a change in the construct itself? Or is the change more attributable to properties of the vehicle, or to properties of the scale of measurement? The item content of the Stanford-Binet IQ tests, for example, differs from one age level to the next. Several different highly g-loaded tests (e.g., Stanford-Binet, Wechsler, Raven) differ in other factors unrelated to g. What exactly has changed--the level of g or the non-g sources of variance? Is a unit change in one range of the measuring scale equivalent to a unit change in another range, that is, are the measurements an interval scale throughout their range? A change in the measurement is not necessarily a change in the level of the construct; it could reflect any one (or a combination) of several different sources of variance in the measurements.
Evidence for an authentic change in the construct g requires broad transfer or generalizability across a wide variety of cognitive performance. Anything less implies changes in lower-order factors, or