The pill that went the wrong way

About this video

A headline says "Coffee drinkers live longer", and thousands of people share it. Before you can decide what it means, you need to know one thing about the study: did the scientists watch people, or did they test something fairly? Two things happening together doesn't show that one causes the other. It could be chance, or a third thing causing both.

In an observational study, scientists watch what people already do, what they eat, how much they move, whether they smoke, and see who gets ill over the years. These studies find links, but people who differ in one habit usually differ in many others too, so a link can come from any of them. In a randomised trial, chance decides who gets something and who doesn't, and then the groups are compared. Because chance made the groups alike, the thing being tested is the only planned difference, so if the groups end up different, that thing is the likely cause.

Beta-carotene is the orange colour in carrots and mangoes. In observational studies, people who ate lots of fruit and vegetables rich in it got lung cancer less often. Perhaps beta-carotene protected them? Scientists in Finland tested it: for 5 to 8 years, 29,133 men who smoked took beta-carotene pills or look-alike pills with nothing in them. The result was a shock: the men taking beta-carotene got about 18% more lung cancer, not less. So the link the watchers found didn't come from the beta-carotene itself, but from something else, the whole food or other things about people who eat lots of fruit and vegetables.

Observational studies are still very useful: they find links worth testing, and some questions can't be tested in a trial at all, because no one may give people something harmful on purpose. But a link on its own is a reason to ask more, not proof. When you read a health headline, ask: did they watch people, or test it fairly? And what else is different about these people? This video doesn't say whether coffee is good or bad for you. It goes deeper into cause or coincidence on Knaph, from the Health Level 1 lesson Linked Is Not the Same as Caused.

Transcript

1. 0:00Coffee drinkers live longer, says the headline. Did the scientists watch people, or test it?
2. 0:08An observational study watches what people already do. But people who differ in one habit differ in many. In a randomised trial, chance decides who gets what. The groups are alike, so the thing tested is the likely cause.
3. 0:26People who ate lots of fruit and vegetables rich in beta-carotene got lung cancer less often. Perhaps it protects?
4. 0:36In Finland, over twenty nine thousand men who smoked took beta-carotene pills, or look-alike pills, for years. Those on beta-carotene got about eighteen percent more lung cancer.
5. 0:52So the link came from something else. The whole food, or other things about those people. Only the fair test could show it.
6. 1:02Watching still finds links worth testing. But for a headline, ask two questions. Did they watch, or test? And what else is different about these people?
7. 1:16Watching finds links. A fair test shows a cause.