You read about an 85-year-old who just climbed a big mountain. The article says he had a big dish of ice cream every day for the last 5 years.
What goes through your mind?
Eating ice cream must help develop your athletic powers.
Eating ice cream helps you resist disease.
More people who want to be like this 85-year-old should eat ice cream.
Get the idea? It may sound far-fetched. But instead of “eating ice cream for the last 5 years” substitute “exercised vigorously for the last 5 years.” Now that seems more plausible.
What else could go through your mind?
How about, “He was lucky.”
Or how about, “This is the fallacy of incomplete evidence.”
What *should* go through your mind is, “How many people ate a big dish of ice cream” or “exercised rigorously” and didn’t get those results? If we don’t have an answer, we’re only hearing half the story.
It’s called cherry-picking, or the fallacy of incomplete information.
Let’s stay with the vigorous exercise, which seems more plausible.
Suppose we have a group of 1000 people. Half of them exercised vigorously; half of them were couch potatoes. We’re assuming the groups were chosen randomly and also assigned randomly to one group or another.
After 5 years, 70% of the exercisers were enjoying happy, active lives. So were 50% of the couch potatoes.
Wnat can we say?
Exercising vigorously moves the needle a little: your absolute difference is 20%. If you wanted to play games with numbers, you could say that the relative increase was 40% (20/50).
But you couldn’t say that exercise offered a guarantee. Thirty percent of the exercisers didn’t make it.
You couldn’t say that failure to exercise was a killer. Fifty percent of the non-exercisers did just fine.
We see this all the time in the popular press. We read about an 85-year-old who lived right, exercised, ate well, and now is an athlete. So we say, “What did they do? Maybe we should too.”
Without a controlled experiment, we can’t say anything. Lots of people exercise and don’t turn out like that superstar in the article.
It’s like reading an article, “Rich people don’t wear expensive watches.” But lots of poor people don’t wear expensive watches. And lots of rich people wear Rolex watches. Wearing a cheap watch won’t increase the likelihood that you’ll get rich.
You can do everything right and still die of a heart attack. You can do everything wrong and die in your sleep at a ripe old age.
No guarantees.
