It is given that only 0.1 % of a large population have COVID infection. In this population, the reliability of COVID RTPCR-test is specified as follows :

For persons having COVID, 90% of the test detects the disease but 10% goes undetected. For persons not having COVID, 99% of the test is judged COVID negative but 1% are diagnosed as COVID positive.

Based on the above informations, answer the question :

The probability of the person tested as COVID positive, given that he is actually having COVID is :

$\frac{9}{10}$

Let A = Person selected has covid

B = Person don't have covid

C = Person have judge report positive

P(covid tested/has covid) = 90% $=\frac{90}{100}=0.9$

$P(C/A) = 0.9 = \frac{9}{10}$