For the conclusions drawn from the results of an experiment to have validity, it is essential that the items or patients assigned to treatment and control groups be representative of the same population.
[5] In some experiments, such as many in agriculture[6] or psychology,[7][8][9] this can be achieved by randomly assigning items from a common population to one of the treatment and control groups.
There were a wide variety of ending points for patients such as death, myocardial infarction, stroke, etc.
For example in the ALLHAT trial, Thiazide diuretics were demonstrated to be superior to calcium channel blockers or angiotensin-converting enzyme inhibitors in reducing cardiovascular events in high risk patients with hypertension.
In the ALLHAT study, the clinical control group was not a placebo it was ACEI or Calcium Channel Blockers.