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Old 10-03-2012
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Stats 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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Old 10-03-2012
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I think it is 50...
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Old 10-03-2012
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Default my answer :)

D. 50.....
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Old 10-03-2012
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Quote:
Originally Posted by chux View Post
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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Old 10-03-2012
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it's Uworld Q i guess----D)50
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Old 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
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Old 10-03-2012
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Quote:
Originally Posted by Doc4Step1 View Post
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
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Old 10-03-2012
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Quote:
Originally Posted by venky2600 View Post
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.
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Default 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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