Question:

The example below describes a study that compared three groups of people; the first group drinks mostly wine, the second drinks mostly beer, and the third drinks mostly spirits. This study is comparative but it is not an experiment. Why not?

Example: Moderate use of alcohol is associated with better health. Some studies suggest that drinking wine rather than beer or spirits confers added health benefits. But people who prefer wine are different from those who drink mainly beer or stronger stuff. Wine drinkers eat more fruit and vegetable and less fried food. Their diets contain less fat, less cholesterol, and also less alcohol. They are less likely to smoke. Finally, wine drinkers as a group are richer and better educated than people who prefer beer or spirits. A large study therefore concludes: “the apparent health benefits of wine compared with other alcoholic beverages, as described by others, may be a result of confounding by dietary habits and other lifestyle factors.