Using factor analysis or related statistical methods, it is possible to compute a single common factor that can be regarded as a summary variable characterizing the correlations between all the different tests in a test battery. Spearman referred to this common factor as the "general factor", or simply "g". (By convention, "g" is always printed as a lower case italic.) Mathematically, the "g" factor is "a source of variance among individuals", which entails that one cannot meaningfully speak of any one individual's mental abilities consisting of "g" or other factors to any specified degrees. One can only speak of an individual's standing on "g" (or other factors) compared to other individuals in a relevant population.