Tom Evslin: Average is Not Normal

A six-foot man can easily drown in a river which is “on the average” five feet deep.

In the US each woman averages 1.64 children during her childbearing years. Does that mean that a “normal” woman has 1.64 kids? Of course not. One kid, maybe two, maybe three, maybe more. A fractional kid, however, would be very abnormal.

Every night our local tv weather people say tomorrow’s high temperature is going to be so many degrees above or below normal. But they don’t really mean normal even though that’s what it says on the graphic; they mean “average”. And average and normal aren’t the same thing. Here in New England ten degrees above or below average is still “normal”. It is highly unlikely that the high on any particular day will be exactly the average; not as unlikely as 1.64 children but not likely either.

In a New York Times article on whether the recent rain and snow will cure California’s drought, Peter Gleick, co-founder of and senior fellow at the Pacific Institute, a research organization specializing in water issues, complains “We don’t seem to get average years anymore.” In fact there is scarcely ever a weather year which is average anywhere. This is especially true in California where drought and deluge have alternated since long before humans have had any effect on climate.