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#1
05-14-2015
 USMLE Forums Veteran Steps History: Not yet Posts: 252 Threads: 32 Thanked 40 Times in 27 Posts Reputation: 50
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.

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 ?

#2
05-14-2015
 USMLE Forums Newbie Posts: 2 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10

a c
b d

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).

cheers

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