observational trial is a kind of research
study in which people taking one kind of drug (or other
medical treatment) are compared against people using an alternative
drug or treatment. For example, a study might consider a group
of 100 elderly women, 50 of whom have ever taken hormone
replacement therapy (HRT), and 50 of whom have never taken
HRT. If the incidence of Alzheimer's
disease is lower among the 50 women taking HRT than among
the 50 women who are not taking HRT, then the researchers
might conclude that HRT helps prevent Alzheimer's disease.
Observational trials are most useful for
suggesting a previously unsuspected link between a medication
or lifestyle pattern and a disease (or prevention of a disease).
However, observational trials have several
shortcomings. In clinical
trials, the researcher selects which participants will
receive each kind of treatment. By contrast, in observational
trials, the participants themselves select their treatment,
before the study even starts. This can lead to complications
in interpreting the results. Thus, in the above example, it
may well be the case that HRT prevents Alzheimer's disease.
But it may also be the case that doctors stop prescribing
HRT for women who develop Alzheimer's disease, or that women
with Alzheimer's disease fail to report that they have used
HRT in the past because they have forgotten. If either of
these things are true, then it would appear that more women
in the no-HRT group develop Alzheimer's -- even if there is
no direct connection between HRT and Alzheimer's.
For these reasons, once an apparent relationship
has been detected in observational trials, the next step is
for researchers to conduct clinical trials. Continuing the
example above, the clinical trial might consider 100 randomly-selected
women, give half HRT and half an inactive placebo pill, and
then see whether the women on HRT really are less likely to
develop Alzheimer's disease.
Article : "ESTROGEN
by Catherine E. Myers. Copyright © 2006 Memory Loss and the Brain