8.6 Placebo effect and using controls

The placebo effect is concerned with treatments, so are not directly relevant to observational studies.

However, observational studies can still have a control group, but the individuals are not randomly allocated to the control group.

For example, in the Doll & Hill smoking study (Doll and Hill 1950), two groups were being compared: non-smokers (the control group) and smokers.

Subjects were not allocated to the groups, however, so confounding remains a possibility. Again, the groups in the study can be compared (Example 8.3) to see if the groups are different in other ways.


Doll R, Hill AB. Smoking and carcinoma of the lung. British Medical Journal. 1950;221(ii):739–48.