This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Biostatistics, Subject name: Social & Preventive Medicine
Degree of freedom of a chi square test in contingency table of 2 by 2 is? 
Select and explain. 
A. 1, B. Zero, C. 2, D. 4
A: 1
Answer: A: 1 
Explanation: - for a 2*2 contingency table( two columns and two rows), the degree of freedom is 1. - d.f = (c-1) ( r-1) = (2-1) (2-1) = 1 Reference : Park's textbook of preventive and social medicine, 23rd edition, pg no:852