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#1
05-31-2010
 USMLE Forums Veteran Steps History: 1 + CK Posts: 234 Threads: 63 Thanked 128 Times in 63 Posts Reputation: 158
True Positives and False Positives

A screening test for colon cancer is administered to 1000 people with biopsy -proven colon cancer and to 1000 people without colon cancer. The test results are positive for 250 people of proven cases and 100 of those without colon cancer.The screening test is now to be used on a population of 100,000 with a known prevalence rate of colon cancer of 80 per 100,000. Which of the following is the expected number of true positives and false positives in this population of 100,000 people?

True Positives - False positives

a. 20 - 9992
b. 20 - 89,928
c. 40 - 9992
d. 50 - 89,928
e. 60 - 9992
f. 60 - 89,928
g. 80 - 9992
h. 80 - 89,928

#2
06-01-2010
 USMLE Forums Scout Steps History: Not yet Posts: 73 Threads: 7 Thanked 40 Times in 21 Posts Reputation: 60

#3
06-01-2010
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 36 Threads: 15 Thanked 12 Times in 5 Posts Reputation: 22
TP and FP

Thanks

#4
06-06-2012
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 15 Threads: 1 Thanked 0 Times in 0 Posts Reputation: 10

Ans is B
Colon Cancer No Colon Cancer Total

Test + 20 9,992 10,012

Test - 60 89,928 89,988

Total 80 99,920 100,000
#5
06-06-2012
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 15 Threads: 1 Thanked 0 Times in 0 Posts Reputation: 10

Sorry, I meant A, I tried drawing a 4x4 table but I could not do it. In fact, to answer this question you would need two 4x4 tables, one with the initial information and the other with what you could reason from it.
#6
06-06-2012
 USMLE Forums Addict Steps History: 1 + CK Posts: 180 Threads: 41 Thanked 110 Times in 36 Posts Reputation: 120

i got b)...plz tell the correct answer///
#7
06-06-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580
.

Quote:
 Originally Posted by sachinkumar i got b)...plz tell the correct answer///
hmm... I don't think the author of the post will respond - look at the date of his post...

I got A) but I haven't done that question ever before so if anyone has and knows the answer, please give it us here
#8
06-06-2012
 USMLE Forums Veteran Steps History: 1+CK+CS Posts: 207 Threads: 68 Thanked 85 Times in 57 Posts Reputation: 95

I got A.
First calculate sensitivity and specificity, in this case is 25% and 90%, respectively.

For the case with 10,000 ppl in which 80 ppl definitely have cancer, just apply the sensitivity, 80*25%=20(true positive), and the remaining 99920 ppl are definitely without cancer, and here just apply specificity, 99920*90%=89928, substract 99920 by 89928 and then get the false positive 9992.

 Tags Biostatistics-Epidemiology, Step-1-Questions

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