This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Experimental Epidemiology, Subject name: Social & Preventive Medicine
In a controlled trail to compare two treatments, the main purpose of randomization is to ensure that : 
Select and explain. 
A. The two groups will be similar in prognostic factors., B. The clinician does not know which treatment the subj ects will receive., C. The sample may be referred to a known population., D. The clinician can predict in advance which treatment the subjects will receive.
A: The two groups will be similar in prognostic factors.
Answer: A: The two groups will be similar in prognostic factors. 
Explanation: Ans. is 'a' i.e. The two groups will be similar in prognostic factors The below quote from Park will suitably explain the answer- "Randomization is a statistical procedure by which the participates are allocated into groups usually called 'study ' and 'control 'groups, to receive or not to receive an experimental, preventive or therapeutic procedure, man oeuvre or intervention. ,Randomization is an attempt to eliminate 'bias' and allow for comparability. Theoretically it is possible to assure comparability b matching. But when one matches, one can only match, those factors which are known to be important. There may be other factors which are important but whose effect is not recognized or cannot be determined. By a process of randomization, hopefully these factors will be distributed equally between the two groups. Randomization will give the greatest confidence that the groups are comparable. "About other optionsOptions (b) and (d)The clinician knowing or not knowing about the treatment the subjects will receive, is part of Blinding and not randomization.Option (c)Both the 'study' and 'control' samples are derived from the reference population. This is a step taken prior to randomization.First an experimental or study population is selected from a known population (the target or reference population). Later this study population in divided into study and control groups by Randomization.