anchoring and body counts

Anchoring has been in the news a lot lately. I wrote about it recently when it was revealed that anchoring was instrumental in one of the best approaches for the Netflix prize. In this month’s issue of The Atlantic, Megan McArdle writes about how anchoring has affected the estimates of the number of casualties in Iraq due to the invasion. Apparently, the first estimate of the number of casualties–no matter how inaccurate–influences the amounts of subsequent estimates of the number of casualties. So far, the estimates have varied from 81,020 to 1 million. The number most frequently cited is from a Lancet article that claims that 601,027 Iraqi deaths have resulted from the conflict, although this estimate is problematic. McArdle writes about the psychology that influences these estimates:

We anchor most strongly on the first number we hear, particularly when it is shocking and precise—like, say, 601,027 violent deaths in Iraq. And even when such a number is presented only as a central estimate in a wide range of statistical possibilities (as the Lancet study’s figure was), we tend to ignore the range, focusing instead on the lovely, hard number in the middle. Human beings are terrible at dealing with uncertainty, and besides, headlines seldom highlight margins of error.

Once people make an estimate, they have a strong tendency to confirm it. If I ask you whether it is plausible that there are 600,000 Canada geese in Chicago, your thought process might go something like this: Big lake … a lot of parks … very near Canada … OK, sure. Once you’ve said yes to that 600,000 figure, psychological studies show, you’ll continue “recruiting evidence” for it, perhaps noticing an article on a goose refuge near the city. Eventually you’ll wind up surrounded by a little army of facts that support the theory. What most people don’t do is look for ways to falsify it: Shouldn’t the geese still be in Florida at this time of year?

[E]ven if “only” 150,000 people have been killed by violence in Iraq, that’s a damn high price. Conversely, few of the study’s supporters expressed much pleasure at the news that an extra 450,000 people might be walking around in Iraq. After a year and a half of bitter argument, all that anyone seemed interested in was proving they had been right.

For more on estimating casualties, check out the OR/MS Today article by Doug Samuelson about the new book Statistical Methods for Human Rights ( Jana Asher, David Banks, and Fritz Scheuren, eds., Springer, 2008).


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