Essential Evidence-based Medicine, Second Edition (Essential ...

a person’s blood pressure will be higher when the doctor takes it and lower when
taken later by a machine, a non-physician, or repeatedly by their own physician.
Some of this effect is due to the stress engendered by the presence of the doctor;
as a patient becomes more used to having the doctor take their blood pressure,
the blood pressure decreases.
Unidentified parallel interventions may occur on the part of the physician,
health-care giver, investigator, or patient. This includes things such as unconscious
or conscious changes in lifestyle instituted as a result of the patient’s medical
problem. For example, patients who are diagnosed with elevated cholesterol
may increase their exercise while they also began taking a new drug to help lower
their cholesterol. This can result in a greater-than-expected rate of improvement
in outcomes both in those assigned to the drug and in the control or placebo
group.
The reader’s goal is to differentiate the true treatment effect from the perceived
treatment effect. The true treatment effect is the difference between the
perceived treatment effect and the various types of placebo effect as described
above. Studies should be able to differentiate the true treatment effect from the
perceived effect by the appropriate use of a control group. The control group is
given the placebo or a standard therapy that is equivalent to the placebo since
the standard therapy would be given regardless of the patients’ participation in
the study.
A recent meta-analysis combined the results of multiple studies that had
placebo and no-treatment arms. They compared the results obtained by all the
patients in these two groups and found that the overall effect size for these two
groups was the same. The only exception was in studies for pain where an overall
positive effect favored the placebo group by the amount of 6.5 mm on a 100-mm
pain scale. 4 As demonstrated by previous pain studies, this difference is not clinically
significant.
4 A. Hróbjartsson & P. C. Gøtzsche. Is the placebo powerless? An analysis of clinical trials comparing
placebo with no treatment N. Engl. J. Med. 2001; 344: 1594–1602.
Hypothesis testing 119