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#1




Biostat question for the day
A test has just become available to detect a new strain of hepatitis virus. In early trials 1000 people were enrolled to undergo testing. Four hundred participants had a positive initial test, but confirmatory testing showed that only 200 people were truly infected. The remaining participants were negative by both experimental testing and confirmatory testing.
What is the specificity of this new test? 40% 50% 70% 75% 80% 90% 100%
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shahacmc27 (10012014) 
#2




TP= 200
FP= 200 TN= 800 Specificity= TN/FP+TN 800/200+800=80%
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#3




OK so the FN is Zero.
Also, for the new test TN is 600. So, Specificity is 600/200+600=75%
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dr mumble (08262014) 
#4




I think it ll be 80%

#5




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#6




Well i m not sure but
If total 1000 subject 400 r positive, out of that 200 TP. AND 200 FP 1000 minus 200 = 800 are TN SO SPECIFICITY TN/TN+FP 800/800+200 800/1000= 0.8, 80% 
#7




You guys are doing very well
More answers please. Then I give final answer!
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#8




I calculated by your way too but how did u assume FN is zero??

#9




Well now i m confuse between both methods

#10




600 were negative and also did not have disease. So, TN = 600
Out of the 400 who were positive, 200 were falsely positive. So, FP = 200 Specificity is out of all the disease free how many got a negative result or were truly negative. All disease free = 800, True negative = 600 Specificity = 600/800 = 3/4 = 75% Thanks for the question. 
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dr mumble (08262014) 
#11




There is no need to calculate FN as it is not needed for the calculations.
Specificity by definition does not take into account false negatives. Think of it this way to understand the concept: Specificity (true negative rate) is defined as the proportion of negatives which truly does not have the disease (e.g. the percentage of healthy people who are correctly identified as not having the disease). The last sentence is telling you that the remaining (1000400=600) were negative by both the new test as well as the confirmatory test. This means that the TN has to be 600. Also, since the total of all four has to be 1000 it leaves FN as Zero.
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A man doesn't know what he knows until he knows what he doesn't know. “What is man? He's just a collection of chemicals with delusions of grandeur.” 
The above post was thanked by:  
Dr.ann (08242014) 
#12




Tp=200
fp=200 tn=1000400=600 specificity=tn/tn+fp =600/600+200 =600/800 =75% plz tell the correct answer.! 
#13




Well, guys answer is 75% already explained by earlier posters!
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#14




600\800 = 75% is the correct answer

#15




200. 200
0. 600 So specificity is 600/600+200x100 is 75% 
#16




By 2x2 table
Disease +  Test result + 200(a) 200(b)  0 (c) 400(d) As there were no individuals with negative results in screening test who had positive confirmatory test result so c=0 Specifiity=d/c+d 400/200+400=.75 or 75% 
#17




TN=1000400=600
FP=200 >specificity=TN/TN+FP >600/600+200=0.75=75% 
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