Feel the burn
Q: What did you have for lunch?
A: Sichuan-style dry-fried green beans
Q: Because of the health benefits of spicy food?
A: Uh.. no?
Q: “Those who eat spicy foods every day have a 14 per cent lower risk of death than those who eat it less than once a week.” Didn’t you see the story?
A: I think I skipped over it.
Q: So, if my foods is spicy I have a one in seven chance of immortality?
A: No
Q: But 14% lower something? Premature death, like the Herald story says?
A: The open-access research paper says a 14% lower rate of death.
Q: Is that just as good?
A: According to David Spiegelhalter’s approximate conversion formula, that would mean about 1.5 years extra life on average, if it kept being true for your whole life.
Q: Ok. That’s still pretty good, isn’t it?
A: If it’s real.
Q: They had half a million people. It must be pretty reliable, surely?
A: The problem isn’t uncertainty so much as bias: people who eat spicy food might be slightly different in other ways.Having more people doesn’t help much with bias. Maybe there are differences in weight, or physical activity.
Q: Are there? Didn’t they look?
A: Um. Hold on. <reads> Yes, they looked, and no there aren’t. But there could be differences in lots of other things. They didn’t analyse diet in that much detail, and it wouldn’t be hard to get a bias of 14%.
Q: Is there a reason spicy food might really reduce the rate of death?
A: The Herald story says that capsaicin fights obesity, and the Stuff story says bland food makes you overeat
Q: Didn’t you just say that there weren’t weight differences?
A: Yes.
Q: But it could work some other way?
A: It could. Who can tell?
Q: Ok, apart from your correlation and causation hangups, is there any reason I shouldn’t at least use this to feel good about chilis?
A: Well, there’s the fact that the correlation went away in people who regularly drank any alcohol.
Q: Oh. Really?
A: Really. Figure 2 in the paper.
Q: But that’s just correlation, not causation, isn’t it?
A: Now you’re getting the idea.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »