This study provides more evidence that much of medicine is based on the body's own healing abilities being marshalled through a placebo effect. In this case, the "treatment" consisted only of informing hotel housekeepers that their work amounted to a substantial amount of physical exercise which provided health benefits. The control case was only different in that they were not provided the same informational program that explained the work they were already doing was healthy exercise.
The group informed of the health benefits of their job lost weight, improved their blood lipids profile, and showed a number of other beneficial health effects all of which were statistically significant changes versus the control group. Three of the measurements showed strong statistical significance to the .001 level, or 999 out of 1000 against the null hypothesis. And remember, the only difference is that one group was informed that the work they were already doing counted as "healthy exercise" and the other was not.
Hat tip: Robin Hanson.
Update: Robin also references this study showing that most of the effects from antidepressants are placebo effects.