This graphic shows when random allocation (randomization) should take place during the study process. Basically, the researcher starts with a defined population that is randomized to a new treatment or current treatment. The subjects are then followed to see how many improve for the new treatment and current treatment. Sometimes a treatment may be compared to no treatment, but this is very rare. If there is already a treatment in place, it is best to compare the new treatment to a treatment already in place.

Randomization increases the likelihood that the groups will be comparable in regard to characteristics that may affect prognosis. Such characteristics include sex, age, race and severity of disease.

Then again randomization isn’t a guarantee of comparability because chance does play a role in the process. In the long run, the groups tend to be similar. Finally, randomization is ethical only when we do not know whether one treatment is better than the other.