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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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