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When a clinical trial is technically well-designed, is conducted properly,
and analyzed correctly, it can be said that there is high
internal
validity. This means that the results are valid within the context
of the clinical trial and any differences can be attributed to the treatment
under investigation. However, the objective of a trial is usually to
generate inferences about the efficacy of a treatment among a larger group of
similar patients. If we were investigating the effect of a new treatment on
survival in disease X but in our clinical trial we were able to include only
a very narrowly-defined group of patients with disease X (perhaps only in
men over the age of 21 who attend a particular clinic), we might be concerned
that the results are not applicable to the general population of patients
with disease X (which might be 50% female, with many patients under 21 years
of age).
By definition, all clinical trials populations are selected, as only those
who agree to participate may be included. Therefore, the representativeness
of the trial population should always be questioned. In general, the results
of clinical trials that are
externally
validmay be said to be generalizable to other patients who meet the
eligibility criteria for the trial.
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