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Statistics: Decisions Through Data

UNIT 16: The Question of Causation

Summary of the Video

Does smoking really cause lung cancer and other health problems? Much of the video looks at the history of how the evidence linking smoking to lung cancer grew over time. Smoking and health is the vehicle for discussing broader issues about how to get good evidence for causation.

First, cartoons illustrate some light examples to remind us that a close association does not always arise from a cause-and-effect relation. Louisiana has been gaining people and at the same time its coastal regions are sinking—but it's not the case that all those people are causing the state to sink. Ice cream sales and deaths from drowning tend to go up and down together. Does eating ice cream cause drowning? No—both are higher in hot weather. They show a common response to a lurking variable in the background.

So the fact that there is a strong association between smoking and lung cancer isn't enough to show that smoking actually causes lung cancer. The best way to make a case for causation is to do an experiment . Another cartoon suggests an experiment that would clear up the issue of smoking and health: force half of a group of newborns to smoke their entire life, and forbid the other half to smoke. Wait to see how many in each group get lung cancer. That's hopelessly unethical. Given that we can't do an experiment, how can we get evidence for causation?

We tour the history of the controversy over smoking and health, with old cigarette ads showing how tobacco interests responded. As lung cancer rose after smoking became popular, some surgeons noticed that most of their lung cancer patients were smokers. This observation led to retrospective studies that compared people with and without lung cancer, looking for big differences in background or habits. Smoking stood out. Retrospective studies look back in time. Prospective studies look forward. A prospective study follows a group of smokers and a similar group of non-smokers to observe what they die of. Many more smokers die of lung cancer. The association between smoking and lung cancer stood up in many different studies in different places and with different kinds of people. The association can't be explained by a fault in one study or by the special nature of one group of people.
But even a prospective study is not an experiment. People choose whether to smoke. If something else—perhaps heredity—leads people to smoke and also leads to lung cancer, smoking should not be blamed for this common response. Animal experiments are possible, and they showed that cigarette smoke does contain substances that cause cancer in animals. The experiments also confirmed a dose/response relationship: more smoke causes more cancer. Even though causation can't be directly demonstrated in people because experiments are not possible, we know that cigarette smoke causes cancer in animals.

Between 1940 and 1960, while this research was going on, per person cigarette consumption doubled. In 1962, the Surgeon General assembled a group of experts to review the entire issue. They concluded that there was excellent evidence that cigarette smoking did in fact cause lung cancer. Since the Surgeon General's report, smoking has been under attack and has declined considerably.

(Although the video stops with the Surgeon General's report, there is now even more evidence. Lung cancer was once almost unknown in women, but it rose rapidly after women began to smoke, with about a 30-year lag because lung cancer develops slowly. Heredity can't explain this. Nor can heredity or most other lurking variables explain the danger of “passive smoking.” People who don't smoke but who live with smokers are more likely to get lung cancer than people who don't live with smokers. The evidence that smoking causes lung cancer is about as strong as non-experimental evidence can be.)


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