>>11679242COVID-19 affects 0.1% of the population.
A test for it is 99% accurate.
You test positive.
What is the chance you are actually infected?
A. 0.1%
B. 9%
C. 32%
D. 99%
ROUND #1
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100000 people tested
0.1% are sick = 100 people
99.9% are healthy = 99900 people
test accuracy 99%
of the 100 sick, 99 will test as positive
of the 100 sick, 1 will test as negative (FN)
of the 99900 healthy, 999 will test as positive (FP)
of the 99900 healthy, 98901 will test as negative
99/(99+999)=99/1098=0.09016393443
B. 9%
ROUND #2
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1098 people tested
9% are sick = 99 people
91% are healthy = 999 people
test accuracy 99%
of the 99 sick, 98 will test as positive
of the 99 sick, 1 will test as negative (FN)
of the 999 healthy, 10 will test as positive (FP)
of the 999 healthy, 989 will test as negative
98/(98+10)=98/108=0,907407407 ? 91%
