In clinical trials, the major purpose of randomization is to:

Randomization is a statistical procedure by which the selection bias is reduced and thus the paicipants are allocated into study and control groups to proceed with comparability. By randomization, there is equal distribution of factors between the two groups and thus comparison is allowed. Although randomization makes the groups comparable, it does not 'ensure' comparability. Also double blinding is not facilitated by randomization. Ref: Park's Textbook of Preventive and Social Medicine, 19th Edition, Pages 77-78