maths a level probability question

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abovethecl0uds
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#1
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#1
i'm stuck on this question.. any help would be appreciated! i tried drawing a tree diagram but i don't think you're supposed to draw one for this question
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mqb2766
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A tree diagram might help in getting the joint probabilities, ,so upload what you've done so far and what are you stuck with?
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abovethecl0uds
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#3
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sure! this is my tree diagram
i thought this was a conditional probability q so i tried to find the probability of a person getting a positive test given that they have the virus:
0.98 / 0.001
but this is greater than 1 so it's obviously wrong
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mqb2766
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#4
Youve got to write things down clearly. You don't have to use a tree to get the joint probabilities, but it would help. Youve not included the probabilities on the first level, and I dont know where the 0.98/0.001 comes from?
Write down the conditional probability you want to find and use the conditional expression to work out which terms you need to use.
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abovethecl0uds
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#5
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ah ok i've drawn a new tree diagram and i think i've got the answer?
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mqb2766
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#6
Looks good.
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abovethecl0uds
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(Original post by mqb2766)
Looks good.
great thanks so much for the help!!
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mqb2766
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#8
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(Original post by abovethecl0uds)
great thanks so much for the help!!
Its not always necessary, but sometimes an approximate solution is useful for validation. Here
~ 1 person in 1000 has the virus and test positive
~ 10 people in 1000 gives a false positive (tests positive but doesn't have the virus)
So ~ 9% will have the virus given a positive test (1/11). Pretty accurate for these numbers.
Last edited by mqb2766; 3 months ago
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