2.6 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 2.3) to see if the groups are different in other ways.