Imagine that, over the course of a conversation with a friend from work, she makes the following two statements:
- It’s possible that my brother will be coming into town tomorrow
- It’s possible that our boss knows about the affair you had with the intern
(You might also have to imagine a more adventurous romantic life for yourself). Which of these two statements do you think your friend believes is most likely to be true? Let’s make this a poll:
If I did a good job setting up this scenario, I should be able to predict the results of the poll. I’ll get to my prediction in a minute.
First, let’s talk a little about why this question is important. The most obvious application of judgments about probability comes from the field of medicine. We’ve discussed a key problem doctors have in communicating with their patients — many patients don’t understand numerical probability. So if a doctor says, for example, “there’s a 1 percent chance you’ll go blind from this surgery,” many patients will systematically misunderstand what that means.
One possible way to get around this limitation is to use qualitative statements instead of percentages: “it’s extremely unlikely that you’ll go blind from this surgery.” But, as we’ll see, there are problems with this approach as well. Jean-François Bonnefon and Gaëlle Villejoubert asked over 800 people from a broad range of backgrounds to imagine two different medical scenarios:
- Your family doctor tells you that you possibly will develop insomnia soon.
- Your family doctor tells you that you possibly will develop deafness soon.
For each scenario, the respondents attempted to convert the doctor’s qualitative statement into a numeric value, by rating the likelihood that the doctor believed that they had a given percentage chance of developing insomnia or deafness. Here are the results:
When the condition is seen as more severe, people are significantly more likely to think the doctor believes the condition is more likely to develop — even though the doctor used the same words to describe the probability. Bonnefon and Villejoubert believe the “patients” are using a similar process to how you might respond to your friend telling you your boss “possibly” knows about your affair. Since this news is possibly embarrassing to you, you believe that your friend is simply using the word “possibly” to help you save face. When your friend says her brother is “possibly” coming to town, there’s no potential embarrassment or harm to you, so you take it as a legitimate assessment of probability.
Bonnefon and Villejoubert argue that the doctor-patient relationship is similar: your doctor uses “possibly” in the case of deafness to “soften the blow,” not as an honest assessment of probability. When the condition isn’t as severe (insomnia), you’re more likely to believe it to be an accurate assessment of your chances of developing the condition. Their data backs this argument as well: they asked patients why the doctors chose the word “possibly,” and in the case of deafness, they were significantly more likely to say that it was a case of “face management,” rather than a judgment of probability.
So if some patients don’t understand numerical probability, and if qualitative statements of probability can be misinterpreted, what’s the conscientious doctor to do? With younger, more educated patients, it seems that numerical statements are the clear winner. But with less-educated and older patients, it may be extremely difficult to strike the right balance between accuracy and understanding.
Bonnefon, J., & Villejoubert, G. (2006). Tactful or doubful? Expectations of politeness explain the severity bias in the interpretation of probability phrases. Psychological Science, 17(9), 747-751.