This isn't quite right - the selection of patients is usually not explicitly looser in phase 3 trials. Speaking from experience in oncology, in early phase tri

This isn't quite right - the selection of patients is usually not explicitly looser in phase 3 trials. Speaking from experience in oncology, in early phase trials, the patients are younger, healthier, wealthier, better educated, have less burden of disease and also probably have better disease biology. This is partly because doctors have to decide to refer a patient for a trial, which may be at another hospital or even another city - they select the 'best' patients for these purposes. This is just being practical in many ways - an 80 year old in a wheel chair isn't going to be able to spend 2 hours commuting every day to enrol in a phase I trial. A single mother with 4 kids working 3 jobs is also going to struggle to take multiple days off work for the trial. Patients that self-select for early phase trials are also a distinct group, generally healthier, wealthier and better educated. There is a subtle point about disease biology - many early phase trials start in populations that have run out of other treatment options. You actually need a relatively indolent cancer to make it to this stage. Very aggressive tumours will kill the patient before they can get through 1 or max 2 lines of therapy to even be eligible for the trial. In phase 3 trials, these selection biases are less prevalent (that's the point) and randomisation also takes out the effect of patients that were going to do relatively well anyway receiving a treatment that may not be effective.