This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Contraceptive efficacy is generally assessed by measuring the number of unplanned pregnancies during use of a contraceptive method. Best index for measuring contraceptive efficacy is: 
Select and explain. 
A. Pearl index, B. Life-table analysis, C. Chandler's index, D. Quetlet index
B: Life-table analysis
Answer: B: Life-table analysis 
Explanation: The two methods that are used to measure contraceptive efficacy are the Pearl index and Life-table analysis. The Pearl index is usually based on a lengthy exposure (usually 1 year) and, therefore, fails to accurately compare methods at various durations of exposure. This limitation is overcome by using the method of Life-table analysis. Life-table analysis calculates a failure rate for each month of use. A cumulative failure rate can then compare methods for any specific length of exposure. Ref: Park's Textbook of Preventive and Social medicine, 19th edition, Page 406.