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A terrible disease sweeps around the world, luckily only affecting 1 [#permalink]
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Explanation:

If a person has a positive report then, there is 99% of chance that he is actually positive
i.e. He will be the 1 out of those 10000 tested.

So, Probability = \(\frac{1}{10000}(0.99) = \frac{99}{1000000}\)

There is also a 1% chance that he was tested fake positive
i.e. He will be the 9999 out of those 10000

So, Probability = \(\frac{9999}{10000}(0.01) = \frac{9999}{1000000}\)

Thus required Probability = \(\frac{0.000099}{(0.000099 + 0.009999)} = \frac{0.000099}{0.010098} = 0.009803\)

Col. A: 0.009803
Col. B: 0.009803

Hence, option C
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Re: A terrible disease sweeps around the world, luckily only affecting 1 [#permalink]
KarunMendiratta wrote:
Explanation:

If a person has a positive report then, there is 99% of chance that he is actually positive
i.e. He will be the 1 out of those 10000 tested.

So, Probability = \(\frac{1}{10000}(0.99) = \frac{99}{1000000}\)

There is also a 1% chance that he was tested fake positive
i.e. He will be the 9999 out of those 10000

So, Probability = \(\frac{9999}{10000}(0.01) = \frac{9999}{1000000}\)

Thus required Probability = \(\frac{0.000099}{(0.000099 + 0.009999)} = \frac{0.000099}{0.010098} = 0.009803\)

Col. A: 0.009803
Col. B: 0.009803

Hence, option C

Could you please help me understand if we already considered true positive then why do we need to consider false positive?
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Re: A terrible disease sweeps around the world, luckily only affecting 1 [#permalink]
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koala wrote:
KarunMendiratta wrote:
Explanation:

If a person has a positive report then, there is 99% of chance that he is actually positive
i.e. He will be the 1 out of those 10000 tested.

So, Probability = \(\frac{1}{10000}(0.99) = \frac{99}{1000000}\)

There is also a 1% chance that he was tested fake positive
i.e. He will be the 9999 out of those 10000

So, Probability = \(\frac{9999}{10000}(0.01) = \frac{9999}{1000000}\)

Thus required Probability = \(\frac{0.000099}{(0.000099 + 0.009999)} = \frac{0.000099}{0.010098} = 0.009803\)

Col. A: 0.009803
Col. B: 0.009803

Hence, option C

Could you please help me understand if we already considered true positive then why do we need to consider false positive?


Because the total outcomes would be all possibilities of getting a positive result i.e. 99% positive (Actual) + 1% positive (Fake)
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Re: A terrible disease sweeps around the world, luckily only affecting 1 [#permalink]
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Re: A terrible disease sweeps around the world, luckily only affecting 1 [#permalink]
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