Much of psychometric theory is merely an extension, an elaboration, or a generalization
of these concepts. Almost all tests are composites of items or subtests. An understanding
how to decompose test variance into its component parts, and conversely, an understanding
how to analyze tests as composites of items, allows us to analyze the meaning of tests. But
tests are not merely composites of items. Tests relate to other tests. A deep appreciation of
the basic Pearson correlation coefficient facilitates an understanding of its generalization to
multiple and partial correlation, to factor analysis, and to questions of validity.