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#1




Calculating False Negatives from Specificity and Sensitivity!
A research for diagnosing colorectal cancers has sensitivity of 75% and specificity of 90%. There are 200 patients who truly had colon cancer in the study and 300 who truly did not.
How many false negatives were in the study? A. 60 B. 80 C. 67 D. 50 E. 100
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#2




:)
I think it is 50...

#3




my answer :)
D. 50.....

#4




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#5




it's Uworld Q i guessD)50

#6




This is my first time seeing this question cos I have not done Biostatics in UWorld...
Before looking at the answers I did it my way.got 50 the answer.I don't know if my way of doing it was right or I just got the answer right. So if we make a 4x4 table. TP=200 FP=? FN=? TN=300 Now Sensitivity is 75%...So if TP is 200 which should be 75% + (25% off FN)...so 25% of 200 is 500. Confusing but I got it right...Just want to know if this is the right way.Or I have just become an inventor? lolz 
#7




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according to uworld not TP=200....,,..TP+FN=200 so ,sensitivity = TP/TP+FN..... 0.75= TP/ 200 TP= 200* 0.75 TP=150 FN= 200150=50 i also got doubt while doing it in uworld ,why they considered TP+FN =200.......i think that's the reason chux posted here.......i also wanted to know the explanation 
#8




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Last edited by Doc4Step1; 10032012 at 03:15 PM. 
#9




D is correct
its actually uworld question.
@vencky,its so confusing but they are right 200 are truely positive; which means both the True positives and the False Negatives because the FN actually have the dx but the screening test could not detect it.
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