Dimensions, factors, and traits are to be contrasted with categories, taxa, and types. Dimensions can be thought of as differences in degree, whereas categories can be thought of as differences in kind (Meehl, 1992). Differences in degree can be large or small--conceptually, infinitely small, as is the case with real numbers on a number line. Differences in kind do not yield to linear conceptualization--for example, blood type is difficult to conceive as lying along any continuum. Once the distinction between traits and types is understood, one can still ask whether it is important to draw the distinction conceptually, and whether it is possible to detect the distinction empirically.