In which I attempt to explain a phenomenon I do not understand. Within an arbitrary time limit.
What is it?
The placebo effect relies on belief, expectation. Sometimes, “placebo effect” is used as shorthand for “any improvement not attributable to an active intervention” – this improvement could be due to a number of factors. When used in this way, the term does cover the true placebo effect, but it also includes the following:
Spontaneous improvement, fluctuation of symptoms, regression to the mean, additional treatment, conditional switching of placebo treatment, scaling bias, irrelevant response variables, answers of politeness, experimental subordination, conditioned answers, neurotic or psychotic misjudgment, psychosomatic phenomena, misquotation, etc. [Kienle and Kiene, 1997]
The true placebo effect is what Moerman and Jonas referred to as the meaning response.
Ernst and Resch tried to differentiate between the perceived and the true placebo effect in a 1995 paper in the BMJ.
How important is it?
HK Beecher wrote an influential paper in 1955 titled “The powerful placebo”. A 1997 paper by Kienle and Kiene asked whether Beecher’s powerful placebo was fact or fiction. [PDF here.] They claimed that “awareness of Beecher’s mistakes and misinterpretations is essential for an appropriate interpretation of current placebo literature”.
Scientists love arguing about ideas, and the placebo effect is a good arena for these intellectual gladiators.
Hróbjartsson and Gøtzsche asked Is the Placebo Powerless? in a paper published in NEJM and found little evidence in general that placebos had powerful clinical effects.
Disagreement came from (among others) Wampold et al – responded to by Hróbjartsson and Gøtzsche here. Hunsley and Westmacott offer this, in which they note that “Meta-analytic results reported by the two sets of authors are nearly identical”.
Hunsley and Westmacott go on to argue that placebo effects do exist and cannot be dismissed as unimportant – but also that it is reasonable to describe the effect size for placebos in medicine as small.
The placebo effect is real, is often misunderstood, and there is some controversy as to just how powerful it is.
It’s also pretty damn interesting.
A couple of interesting links:
The bias of compliance: “…people who take their placebo regularly are just different than the others. The rest is a little speculative. Maybe they take better care of themselves in general. But this compliance effect is quite a big effect.”
Edit, 2nd October 2011: At least in this study, it appears that a placebo effect can operate when the outcome of interest is self-rated improvement, but not when an objective outcome is used. This finding is in accordance with what Hróbjartsson and Gøtzsche originally reported…