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The Renzettis live in a small house at 84 Chestnut Avenue. Frank Renzetti is forty-four and works as a bookkeeper for a moving company. Mary Renzetti is thirty-five and works part-time at a day care. They have one child, Tommy, who is five. Frank’s widowed mother, Camila, also lives with the family. My question: How likely is it that the Renzettis have a pet? To answer that, most people would zero in on the family’s details. “Renzetti is an Italian name,” someone might think. “So are ‘Frank’ and ‘Camila.’ That may mean Frank grew up with lots of brothers and sisters, but he’s only got one child. He probably wants to have a big family but he can’t afford it. So it would make sense that he compensated a little by getting a pet.” Someone else might think, “People get pets for kids and the Renzettis only have one child, and Tommy isn’t old enough to take care of a pet. So it seems unlikely.” This sort of storytelling can be very compelling, particularly when the available details are much richer than what I’ve provided here. But superforecasters wouldn’t bother with any of that, at least not at first. The first thing they would do is find out what percentage of American households own a pet. Statisticians call that the base rate—how common something is within a broader class. Daniel Kahneman has a much more evocative visual term for it. He calls it the “outside view”—in contrast to the “inside view,” which is the specifics of the particular case. A few minutes with Google tells me about 62% of American households own pets. That’s the outside view here. Starting with the outside view means I will start by estimating that there is a 62% chance the Renzettis have a pet. Then I will turn to the inside view—all those details about the Renzettis—and use them to adjust that initial 62% up or down. It’s natural to be drawn to the inside view. It’s usually concrete and filled with engaging detail we can use to craft a story about what’s going on. The outside view is typically abstract, bare, and doesn’t lend itself so readily to storytelling. So even smart, accomplished people routinely fail to consider the outside view. The Wall Street Journal columnist and former Reagan speechwriter Peggy Noonan once predicted trouble for the Democrats because polls had found that George W. Bush’s approval rating, which had been rock-bottom at the end of his term, had rebounded to 47% four years after leaving office, equal to President Obama’s. Noonan found that astonishing—and deeply meaningful.9 But if she had considered the outside view she would have discovered that presidential approval always rises after a president leaves office. Even Richard Nixon’s number went up. So Bush’s improved standing wasn’t surprising in the least—which strongly suggests the meaning she drew from it was illusory. Superforecasters don’t make that mistake. If Bill Flack were asked whether, in the next twelve months, there would be an armed clash between China and Vietnam over some border dispute, he wouldn’t immediately delve into the particulars of that border dispute and the current state of China-Vietnam relations. He would instead look at how often there have been armed clashes in the past. “Say we get hostile conduct between China and Vietnam every five years,” Bill says. “I’ll use a five-year recurrence model to predict the future.” In any given year, then, the outside view would suggest to Bill there is a 20% chance of a clash. Having established that, Bill would look at the situation today and adjust that number up or down.
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Philip E. Tetlock (Superforecasting: The Art and Science of Prediction)