Page 182 - Malcolm Gladwell - Talking to Strangers

More on the dilemmas caused by haystack searches: Middle-aged women, in most countries, are
encouraged to get regular mammograms. But breast cancer is really rare. Just under 0.5 percent of
women who get a mammogram actually have the disease. Looking for breast cancer is therefore a
haystack search.
Epidemiologist Joann Elmore recently calculated just what this means. Imagine, she said, that a
group of radiologists gave a mammogram to 100,000 women. Statistically, there should be 480
cancers in that 100,000. How many will the radiologists find? 398. Believe me, for a task as difficult
as reading a mammogram, that’s pretty good.

But in the course of making those correct diagnoses, the radiologists will also run up 8,957 false
positives. That’s how haystack searches work: if you want to find that rare gun in someone’s
luggage, you’re going to end up flagging lots of hair dryers.
Now suppose you want to do a better job of spotting cancers. Maybe getting 398 out of 480 cases
isn’t good enough. Elmore did a second calculation, this time using a group of radiologists with an
extra level of elite training. These physicians were very alert, and very suspicious—the medical
equivalent of Brian Encinia. They correctly identified 422 of the 480 cases—much better! But how
many false positives did that extra suspicion yield? 10,947. An extra two thousand perfectly healthy
women were flagged for a disease they didn’t have, and potentially exposed to treatment they didn’t
need. The highly trained radiologists were better at finding tumors not because they were more
accurate. They were better because they were more suspicious. They saw cancer everywhere.
If you are a woman, which group of radiologists would you rather have read your mammogram?
Are you more concerned about the tiny chance that you’ll have a cancer that will be missed, or the
much larger probability that you’ll be diagnosed with a cancer you don’t have? There’s no right or
wrong answer to that question. Different people have different attitudes toward their own health,
and toward risk. What’s crucial, though, is the lesson those numbers teach us about haystack
searches. Looking for something rare comes with a price.