Editorial: Oasis or Mirage?

In a randomized controlled experiment, investigators assign subjects to treatment or control, for instance, by tossing a coin. In an observational study, the subjects assign themselves. The difference is crucial because of confounding. Confounding means a difference between the treatment group and the control group, other than the causal factor of primary interest. The confounder may be responsible for some or all of the observed effect that is of interest. In a randomized controlled experiment, near enough, chance will balance the two groups. Thus, confounding is rarely a problem. In an observational study, however, there often are important differences between the treatment and control groups. That is why experiments provide a more secure basis for causal inference than observational studies. When there is a conflict, experiments usually trump observational studies. However, experiments are hard to do, and well-designed observational studies can be informative. In social science and medicine, a lot of what we know—or think we know—comes from observational studies. Most studies on smoking are observational. Taken together, they make a powerful case that smoking kills. A great many lives have been saved by tobacco-control measures, which were prompted by observational studies. There are a few experiments; the treatments (e.g., counseling) were aimed at getting smokers to quit. Paradoxically, results from the experiments are inconclusive. Editorial: Oasis or Mirage?