Statistics, more than most other areas of mathematics, is just formalized common sense, quantified straight thinking.
—Paulos (1992, p. 58). Few, 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.