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#1
10-03-2012
 USMLE Forums Addict Steps History: --- Posts: 110 Threads: 4 Thanked 32 Times in 28 Posts Reputation: 42
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
10-03-2012
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:)

I think it is 50...
#3
10-03-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

D. 50.....

#4
10-03-2012
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Quote:
 Originally Posted by chux 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
D. 50

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#5
10-03-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,186 Threads: 119 Thanked 822 Times in 434 Posts Reputation: 832

it's Uworld Q i guess----D)50
#6
10-03-2012
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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
10-03-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,186 Threads: 119 Thanked 822 Times in 434 Posts Reputation: 832

Quote:
 Originally Posted by Doc4Step1 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
luckily you got answer right...but i think uworld solved it differently

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= 200-150=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
10-03-2012
 USMLE Forums Guru Steps History: Not yet Posts: 375 Threads: 17 Thanked 131 Times in 81 Posts Reputation: 147

Quote:
 Originally Posted by venky2600 luckily you got answer right...but i think uworld solved it differently 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= 200-150=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
Thanks venky...I am still gonna do it my way....lolz....All I want is to get it right.

Last edited by Doc4Step1; 10-03-2012 at 02:15 PM.
#9
10-03-2012
 USMLE Forums Addict Steps History: --- Posts: 110 Threads: 4 Thanked 32 Times in 28 Posts Reputation: 42
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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 Tags Biostatistics-Epidemiology, Step-1-Questions

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