Randomized studies of new treatments often exclude the sickest patients that could have benefitet most from the treatment. In particular, there won't be any rea

Randomized studies of new treatments often exclude the sickest patients that could have benefitet most from the treatment. In particular, there won't be any really seriously ill patients in the control group because they will most likely die without treatment. That skews the results of these studies. This is what they try to illustrate with the jumping out of plane study. The result won't be reliable if you don't have a control group that actually might have benefit from the treatment. Cancer patients (jumpers), will or will not get a new type of chemotherapy (parachute). But since it is known that the sickest (hightest jumping hight) certainly will die without treatment (parachute), they decide to only test the treatment on patients with only early stage and benign cancer (close to the ground).