Statistics, more than most other areas of mathematics, is just formalized common sense, quantified straight thinking.
—Paulas (1992, p. 58).
F ew, if any, characteristics of the world are more apparent than variability. We see it everywhere we look. There are, by some counts, tens of millions species of living creatures in the world. When our focus is narrowed to a single species, say our own, variability is still the rule. Excluding identical twins, no two people look exactly alike and even twins can usually be distinguished by people who know them well.
One of the ways in which we cope with diversity is through the process of conceptual categorization. We group things that are similar in certain respects into conceptual classes or categories, give these categories names and then, for many purposes, respond to members of the same category—items with the same name—as though they were identical. Even within categories, however, there is much variability. Though all chairs are chairs, they differ greatly in size, shape, color, and numerous other respects. All snow flakes are six-sided, but no two of them, we are told, have precisely the same crystalline structure. Whereas people share certain characteristics that define their humanity, they differ in height, weight, age, intelligence, hair color, eye color, and countless other less obvious respects.