Implicit in our description of central tendency is the idea that even as one speaks
of some stable component, one acknowledges the fundamental variability inherent in
that process and thus its probabilistic nature. Because of this, we claim that the
notion of an average understood as a central tendency is inseparable from the notion
of spread. That average and variability are inseparable concepts is clear from the
fact that most people would consider talking about the average of a set of identical
values to be odd. In addition, it is hard to think about why a particular measure of
center makes sense without thinking about its relation to the values in the
distribution (e.g., the mean as the balance point around which the sum of the
deviation scores is zero, or the median as the point where the number of values
above equals the number of values below).
Not all averages are central tendencies as we have defined them above. We
could compute the mean weight of an adult lion, a Mazda car, and a peanut, but no
clear process would be measured here that we could regard as having a central
tendency. One might think that the mean weight of all the lions in a particular zoo
would be a central tendency. But without knowing more about how the lions got
there or their ages, it is questionable whether this mean would necessarily tell us
anything about a process with a central tendency. Quetelet described this distinction
in terms of true means of distributions that follow the law of errors versus arithmetic
means that can be calculated for any assortment of values, such as our hodgepodge
above (see Porter, 1986, p. 107).