SDK
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A pts has new screening test for TB..test indicate that pts is well while pts is infected.....
1)To identify all pts infected with TB ,the cut-off point for this test shud be set at the point of highest??
a)senstivity
b)specificty
c)+ve predictive value
d)-ve PV
e)Accuracy
2)If this new screening test has sensitivity of 90% & speficity of 70%..The +ve predictive value of this test is??
a)12.5%
b)25%
c)30%
d)72%
e)90%
Plz explain the answers in detail???
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| mjl1717
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answer-a sensitivity
[But it would have low specificity/the false + results would be unacceptably high.]{Just to add more fuel to the fire-there would be few Type I errors and a large number of Type II errors.
q2 -Ill return with the second answer, using the punnet -like square!
SDK-{You been asking good thought provoking, calculating qs. As you already know no pain no gain} Also congratulations on your 10,000th post 
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| mjl1717
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Using 1000 people:
TP=900
FP=300
FN=100
TN=700
PPV=TP/TP+FP=(900/1200)
=about 75% the closesest # is 72%
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| SDK
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Thnx mjl1717...
But the answer is given as 30/120=25%. BRS page no: 254.
How this occur I donot know.....
Any one has idea about this???
you give a correct equation....90/120....75%...
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| mjl1717
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Just have to say: This type q is what its about. And it seems like there is so
MUCH room for error. Hopefully a statistician will respond. 
If any thing my error MAY be in setting up that punnet-like square.
I try to put the disease on the x axis and test results on the y axis.
Also I made each column( not row) equal 1000 people using your sensitivity/
specificity results.
Also if TP is 90% or 900 the numerator for PPV must be 90% or 900.{As far as I know the the numerators are the same in sensitivity and PPV.}
TP is the # who test positive and have the disease.
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