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the std test to detect std had a sensitivity off 90 % and specificity of 80 % . if a recent cross sectional study found that 1 out of every 10 people in the age group 15 to 19 has the disese, the positive predictive value of this screning test for the people aged 15 tp 19 would be estimated best at ....
21 %
33 %
50 %
67 %
85 %
waiting for the right answe and the explannation...........
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i think 85 %
But somebody plz do the calculation
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sensitivity =90%=TP/(TP+FN),
specificity=80%=TN/(TN+FP)
"if a recent cross sectional study found that 1 out of every 10 people in the age group 15 to 19 has the disese"-----this tells us that TP+FN=10% population of that age group (real disease people)
Suppose the population of that age group is 1000 (use any number you pick, it is going to be the same, cause it is going to be calculated as a ratio) then TP+FN=10%x1000=100
Plot in the sensitivity equation so you get TP/100=90%, so TP=90, therefore FN=100-90=10
Same thing, since among the population of that age group, 100 people have the disease, then 1000-100=900 are healthy people, that's TN+FP=900, plot in the specificity equation:
TN/900=80%, so TN=720, therefore FP=900-720=180
The Q stem asks for PPV=TP/(TP+FP)=90/(90+180)=33%
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Calender,thank you so very much--I have been trying to figure this one out since ages and always got stuck tryiing to calculate the FP.Thanks again.
