are intervention studies using human subjects in which a treatment
(intervention) group is compared to a control group. The control
group is treated the same as the treatment group except that subjects
receive a placebo instead of an intervention.
Subjects in an
intervention study should be randomly assigned to treatment/control
groups, and it is best if neither the subject nor investigator knows
which subjects are receiving treatment and control to avoid bias.
A study in which neither investigator nor subject knows who is receiving
treatment is called a double blind study. Sometimes it is impossible
for the investigators to be kept in the dark about who is receiving
treatment, so single blind studies, in which only the subject is
kept in the dark, are employed.
design" strengthens an intervention study, and negates the
possibility that differences between treatment and control are due
to differences between the individuals in each group. In a crossover
design study, subjects receive both treatment and control conditions
at different periods. A "washout" period between conditions
is employed to make sure the first intervention does not influence
the second intervention. It is best that one group receives treatment
first and the other control first so that order of intervention
is not a confounding factor.