This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Definitions & Concepts, Subject name: Social & Preventive Medicine
[{'content': 'Failure rate of contraceptive method is determined by: \nSelect and explain. \nA. Half life index, B. Pearl index, C. Total feility rate, D. Age specific feility rate', 'role': 'user'}, {'content': "Answer: B \nExplanation: - Pearl's Index: Measures the failure rate/ efficiency of a contraception. PI = Total No. of accidental gestations/ Total months of exposure x 1200 It is expressed per Hundred women years .", 'role': 'assistant'}]