4.2: What is a double-blind trial? What is a placebo?
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This article is from the Scientific Skepticism FAQ, by Paul Johnson Paul@treetop.demon.co.uk with numerous contributions by others.
4.2: What is a double-blind trial? What is a placebo?
A double-blind trial is the standard method for deciding whether or
not a treatment has any "real" effect.
A placebo is a "treatment" that has no effect except through the mind
of the patient. The usual form is a pill containing a little lactose
(milk-sugar), although a bitter-tasting liquid or injections of 1cc
saline can be used instead.
The "placebo effect" is the observed tendency for patients to display
the symptoms they are told to expect.
The problem is that the state of mind of a patient is often a
significant factor in the effect of a course of treatment. All
doctors know this; it is why "bedside manner" is considered so
important. In statistical tests of new treatments it is even more
important, since even a small effect from the state of mind of a small
fraction of the patients in the trial can have a significant effect
on the results. Hence new medicines are tested against a placebo.
The patients in the trial are randomly divided into two groups. One
of these groups is given the real medicine, the other is given the
placebo. Neither group knows which they have been given. Hence the
state of mind for both groups will be similar, and any difference
between the two groups must be due to the drug. This is a blind trial.
It has been found that patients can be unconsciously affected by the
attitude and expectations of the doctor supplying the drug, even if
the doctor does not explicitly tell them what to expect. Hence it is
usual for the doctor to be equally unaware which group is which. This
is a "double blind" trial. The job of remembering which group is
which is given to some administrative person who does not normally
come into contact with patients.
This causes problems for many alternative therapies because they do
something to the patient which is difficult to do in a placebo-like
manner. For instance, a treatment involving the laying-on of hands
cannot be done in such a way that both patient and practitioner are
unaware as to whether a "real" laying on of hands has taken place.
There are partial solutions to this. For instance one study employed
a three-way test of drug placebo, counseling and alternative therapy.
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