UGC NET CSE | December 2015 | Part 2 | Question: 32

Question

Suppose that from given statistics, it is known that meningitis causes stiff neck 50% of the time, that the proportion of persons having meningitis is $\frac{1}{50000}$, and that the proportion od people having stiff neck is $\frac{1}{20}$. Then the percentage of people who had meningtis and complain about stiff neck is:

• A. 0.01%
• B. 0.02%
• C. 0.04%
• D. 0.05%

Answer 1

We can solve this using Bayes formula:

P(B|A) = P(A|B)·P(B) / P(A)

Assume A refer to stiff neck and B refer to probability of meningitis,

P(A|B) = meningitis causes stiff neck =50%= 0.5

P(A)=1/20

P(B)= 1/50000

P(B|A) = 0.5*1/50000/(1/20)= 0.02%

Answer is B