# Guesstimates

Tyler Cowen over at Marginal Revolution points to a new book by Lawrence Weinstein and John Adam, Guesstimation: Solving the World’s Problems on the Back of a Cocktail Napkin.

Cowen asks a sample question: How many people are airborne over the United States at any given moment?

Leaving aside for the moment the obvious fact that it matters whether the moment is mid-morning or middle-of-the night, the first thing that occurred to me was that there were around 6,000 planes in the air when the FAA grounded them on 9/11, so — very roughly — 600,000.

One of the commenters at MR also started with the 9/11 figure, though he recalled it as 4,000. Others explained their estimates reasoning from different assumptions; how many runways active at the several NYC airports, how many daily flights out of Logan airport, how much time does the average American spend in the air in a year?

The thing is, that faced with a blank cocktail napkin, you have to start somewhere, and that means having a mind already well furnished with estimates of other numbers that can be combined to come up with a plausible answer for a novel question.

And it’s not just a parlor game. One of the commenters, giving no rationale, said merely, “10,000.” That isn’t plausible — not if you’ve ever been in a major airport on the day before Thanksgiving, or for that matter ever seen pictures of one on the evening news. People in a democracy are constantly being asked to estimate the plausibility of some policy proposal or other, or to judge the credibility of some politician’s estimates. If they have nowhere to start, their judgment isn’t worth much.

Co-author Weinstein dropped in to the MR comments with a couple of other questions:

How big a landfill would we need to store all our trash for the next century? (And what fraction of the US landmass is that?)

Compare the waste generated per kilometer of horse-drawn carriages and of automobiles.

Cowen says, “This book isn’t for everyone but if you think you might like it you probably will.” Sounds about right.