fuzz123

calculation paper B

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Hey guys, for those that have SPMM mocks - can anyone help to explain the calculation / steps involved in getting the post test probability. for Q24 in the 2019 mock. thanks

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Posted (edited)

On 4/2/2019 at 15:37, fuzz123 said:

Hey guys, for those that have SPMM mocks - can anyone help to explain the calculation / steps involved in getting the post test probability. for Q24 in the 2019 mock. thanks

Hi, I don't have the 2019 Mock but I can try to explain generally if it helps:

 

Prevalence = Pre-test probability

Pre-test odds = The odds of the pre-test event happening vs not happening = Pre-test probability / (1 - pre-test probability)

Post-test odds = Pre-test odds  x Positive likelihood ratio

Side note: Positive LR = Sensitivity / (1 - Specificity)

In case the question asks for negative LR, then formula will be (1 - Sensitivity) / Specificity

So finally,

Post-test probability = Post-test odds / (1 + Post-test odds)

 

In summary, the steps are:

Pre-test probability --> Pre-test odds --> Post-test odds --> Post-test probability

 

Hope it helps. :unsure:

Edited by Alexis

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