This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
The purpose of double blinding in clinical trails is to- 
Select and explain. 
A. Achieve comparability between study and control groups, B. Avoid observer bias, C. Avoid subject bias, D. Avoid observer and subject bias
D: Avoid observer and subject bias
Answer: D: Avoid observer and subject bias 
Explanation: There may be bias in randomized control trials. Randomization cannot guard against these so of bias, nor the size of the sample. In order to reduce these problems, a technique known as blinding is adopted. Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 91