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ber of years, and at the end of the period, those who have developed
the disease are compared with those who have not. In a retrospective
study, the researchers look back, by studying hospital records for ex-
ample, at what has happened in the past, comparing subjects who have
developed the disease with those who have not. In a cohort study and
in a case-control study the subjects are only observed and there is no
intervention such as drug treatment or surgery. A cohort study is a
more reliable method of identifying the cause of disease than a case-
control study. But for proof of cause, a trial is needed.
Trials
In a trial, a group of people who are suffering from a disease are
given a particular treatment. To determine the effectiveness of the
treatment, a controlled trial is performed. Two groups are studied: one
group (the study group) is given the treatment and the other (the con-
trol group) is not. The controls may be given a placebo – something
which seems to be identical to the treatment but which has no effect. If
there is an equal possibility that patients may be selected for the study
group or for the controls, the trial is said to be randomized. A random-
ized controlled trial makes bias – error in a study which influences the
results – less likely. An additional way of removing bias is blinding:
patients don’t know if they are receiving the treatment or the placebo.
If, in addition, the researchers don’t know who is receiving the treat-
ment, the trial is a double blind trial. Randomized controlled trials are
used to test treatments or preventive measures.
Variables
If the subjects in a study are all aged 50, then age is a constant in
the study. If their ages range from 20 to 70, then age is a variable. A
confounding variable is any variable which is associated with both the
disease and the risk factor being studied (for example, smoking in the
case of cooks and lung cancer). If such variables exist there is no way
for the researcher to know whether the difference in the risk factor or
the confounding variable is the one that is truly causing the disease.