How to Make Numbers Count

Posted: July 21, 2011 in Education
Tags: ,

Ask the average American how many soldiers died in the Iraq war.  Rarely do they guess in the ballpark of the correct answer: 4,500.

Ask the average American what percentage of federal spending goes towards development assistance.  I bet they don’t know the answer is less than 1%.

Ask a person how much the first twenty B-2 bombers in the American fleet each cost.  They probably couldn’t tell you whether each cost $600 million, $1.3 billion, or $2.2 billion.

I’ll give you a hint on that last one: two B-2 bombers cost more than the entire budget of the World Food Program, the largest relief agency in the world.

We humans tend to have a tough time with numbers.  It’s not that we cannot perform simple arithmetic but rather that we have a hard time (1) fathoming enormous numbers, (2) understanding the importance of relativity in statistics, and (3) seeing how our sensitivity to numbers varies.  For example, people may not fully understand how many people died in the Holocaust until you explain the number of deaths in terms of buses full of school children, airplanes full of people, or number of states’ populations.  Additionally, many people once believed (and many still do) that the same number of soldiers died in the Vietnam War as in the war in Iraq, when the difference is tenfold.

Which brings me to the purpose of this post… for the sake of promoting honest intellectual discourse, providing an accurate depiction of the scale of numerical values, and avoiding factually incorrect statements, Justin Wolfers put up a post on the Freakonomics blog today with four guidelines for writing and thinking about numbers.  Here are the tips:

Don’t use numbers when words will do. The rhetorical point that Stewart was trying to make is simply that there are a lot of murders.  Too many.  You don’t need numbers to say this.

Don’t use numbers that are hard to comprehend.  We have everyday experience in thinking in dozens.  But we’re hopeless when it comes to millions, billions, or at the other extreme, tiny fractions.  For instance, no one ever made the mistake of saying there are 12,000 eggs in a typical carton. But plenty of journalists confused the $700 billion TARP bailout, describing it as a $700 million plan.

Scale matters: Big (and small) numbers only make sense relative to something else.  Is 15,241 murders a lot for a country the size of the U.S.?  Find a scaling that gives this some meaning (and avoids the artificial precision of 5 significant figures).  Perhaps: Last year around 1 in 20,000 people were murdered.  But how can you get your reader to picture 20,000 people?  Easy, it’s roughly the number of people who attend a typical MLB game.To over-simplify: Look around an average NBA basketball stadium.  If the crowd is representative of the streets, someone in this crowd will be killed this year.

The laugh test: When you really think about your number, does it seem plausible, or is it laughably wrong?  My basketball stadium analogy conveys the true extent of the U.S. homicide problem.  If Stewart had followed this advice, he would have seen that his wrong number implied that 1 in 230 people is killed each year.  Straight away he would realize that he isn’t grieving the murder of one of his Facebook friends every year or two.

The rest of the post can be found here.

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