Placebo-adjusted effect (revision 6)

The placebo-adjusted effect is the difference between the change observed in a treatment arm and the change observed in the comparator arm. It is the quantity a randomised trial is designed to estimate, and it is what can be attributed to the treatment.^[1]

The active-arm change alone conflates the treatment effect with everything else operating during the trial: regression to the mean, the natural course of the condition, the lifestyle intervention both arms received, and the effects of being observed.^[2]

In obesity trials the distinction is substantial. STEP 1 reported −14.9% on semaglutide and −2.4% on placebo, so the placebo-adjusted effect is about 12.4 percentage points — not 14.9.^[3]

A placebo arm in a modern obesity trial is not a no-treatment arm. Participants receive the same lifestyle counselling, the same visit schedule and the same measurement, and the difference between the arms is the treatment alone.^[2][4]

Several distinct phenomena contribute to the change in a placebo arm. Regression to the mean operates because participants are enrolled at a moment when their measurement was high enough to qualify. The trial's own intervention — diet advice, activity advice, regular weighing — has an effect. Observation changes behaviour. And the natural course of a condition may be towards improvement or deterioration.^[2]

The placebo response proper — improvement attributable to expectation — is only one of these, and in objective endpoints such as body weight it is generally small relative to the others.^[2]