docswapna
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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 of the proven case and 100 of those without colon cancer. the screening test is now to be used on a population of 100,000 people with 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 positive False positive
A. 20 9,992
B. 20 89,928
C. 40 9,992
D. 50 89,928
E 60 9,992
F 60 89,928
G 80 9,992
H 80 89,928
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| fox
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A?
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| docswapna
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i am completely hands up for this q. can u plz post how u calculated..
good luck.
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| fox
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Its a pretty straight fwd question. According to the question there are 250 cases positive by test out of a 1000 Bx proven cases. So the true positive is 250/1000 *100 = 25%= sensitivity.
Now there are 100 cases of poitive by the test in 1000 Bx proved negative cases. These are false positives. The true negatives will be 1000-100= 900. Specificity (true neg/total cases X100) is 900/1000= 90%.
Now u have the sensitivity & specificity of the test.
Apply this to the second part of the stem.:
there are 80 known cases of cancer in the 1,00,000 population. So according to the test, true poitive = sensitivity= 25%. 25% of 80= 20. This is the 1st answer.
False pos= 100 - specificity=100%-90%= 10%. Now 10% of total negative= 10% of (1,00,000 - 80)= .1X99,920 =9,992. So this is the second part of the answer. Hence the answer is 20; 9992. Choice A.
Please post the right answer as my biostatistics is 8 years old.!!
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| X-Ray
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Sweet
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| jamdoc
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Gr8 explanation
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| smal
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ya A.
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