Answer:
a) P[A/B] = 0,019 or P[A/B] = 1,9 %
b) P[A- /B-] = 0,9996 or P[A- /B-] = 99,96 %
Step-by-step explanation:
Bayes Theorem :
P[A/B] = P(A) * P[B/A] / P(B)
The branches of events are as follows
Condition 1 real infection 1/300 and not infection 299/300
Then
1.- 1/300 299/300
When the test is done (virus present) 0,9 (+) 0,15 (-)
2.- 299/300
When the test is done ( no virus ) 0,15 (+) 0,85 (-)
Then:
P(A) = event person infected P(B) = person test positive
a) P[A/B] = P(A) * P[B/A] / P(B)
where P(A) = 1/300 = 0,0033 P[B/A] = 0,9
Then P(A) * P[B/A] = 0,0033*0,9 = 0,00297
P(B) is ( 1/300 )*0,9 + (299/300)*0,15
P(B) = 0,0033*0,9 + 0,9966*0,15 ⇒ P(B) = 0,1524
Finally
P[A/B] = 0,00297 /0,1524
P[A/B] = 0,019 or P[A/B] = 1,9 %
b) Following sames steps:
P[A- /B-] = (299/300) * 0,85 / (299/300) * 0,85 + (1/300 * 0,1)
P[A- /B-] = 0,8471 /0,8474
P[A- /B-] = 0,9996 or P[A- /B-] = 99,96 %
