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Old 05-14-2015
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Angry Statistics question my brain friend can you solve this 2x2 table

In a phase 3 clinical trial 1400 subjects, the investigator find that of the 500 subjects who had an MI , 400 tested positive for the serum marker, where as 850 subjects who did not have an MI tested negative for the marker.

If this marker were used to screen patients for recent MI, what is the probability that a person will had an MI given a positive serum protein analyys.

the answer is 400/400+50

how did they come with this answer, I am confused with how to build the table where the number goes where ?

I can see TP = 400 how do i get FN FP TN ?

Can some one tell me how to solve this kind of problems ?
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Old 05-14-2015
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Let your standard 2x2 equal:

a c
b d

Given your scenario above:

500 = a + b
400 = a
850 = d
Thus b = 100
c = 1400-(850+100+400) = 50

Thus it gives you the 2x2:

400 50
100 850

Take the true positive (400) and the false positive (50) to create your "positive predictive value" or "if I have a positive test, what's the chance it's a TRUE positive test"
aka TP/(FP+TP) = 400/(400+50).

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