Because of how clinical trials need to be done for safety reasons and for scientific reasons doctors need to set up criteria for which patients can participate in the study.
A clinical trial is essentially an experiment with willing participants, and doctors need to be able to interpret the data they receive from these trials. This can be very frustrating for patients. But the logic behind the exclusion criteria is that if a patient has extremely advanced cancer and may not make it until the end of the trial, the data could be compromised.
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