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Old 06-01-2011
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Stats Biostatistics #3 - Screening test

A screening test of known sensitivity and specificity is applied to two populations. The prevalence of the disease being screened for is 10% in population A and 1% in population B. Which of the following is true?

a. The percent of all negative tests that have false-negative results is lower in population A than in population B.
b. Specificity is lower in population A than in population B.
c. Reliability is higher in population A than in population B.
d. The percent of all positive tests that have false-positive results is lower in population A than in population B.
e. Sensitivity is higher in population A than in population B.
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  #2  
Old 06-01-2011
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The sensitivity or specificity of a test remains the same in every population ..the false positive or negative no. increase or decrease by changing the cut off value for the test.So i would go with C.
Reliability is higher in population A than B
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Old 06-02-2011
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C) ?
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Old 06-02-2011
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is it c????
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Old 06-02-2011
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Sorry guys, the right answer is not C)
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Old 06-02-2011
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Hello guys,
The right answer is actually D. Allow me to explain:
The sensitivity and the specificity of a test are not affected by the prevalence of a certain disease i.e false negatives and false positives respectively.
The prevalence of disease in A is higher than in B meaning that assuming we don't know the specificity and sensitivity and assuming they are both the same whether in regards to groups A or B, A will have a higher PPV; in other words, those who test positive will truly have the disease is higher than in A than in B (this is what PPV is). Since B has a lower prevalence, NPV will be higher in B than A; meaning: Of those who test negative, more of those in group B than A will be truly negative when compared to a gold standard. PPV is usually increased by a higher specificity and a higher prevalence of disease, whereas NPV is increased by increased sensitivity and a lower prevalence of disease.
Hope this helps
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Old 06-02-2011
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@add1

you're absolutely right
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Old 06-04-2011
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Quote:
Originally Posted by add1 View Post
Hello guys,
The right answer is actually D. Allow me to explain:
The sensitivity and the specificity of a test are not affected by the prevalence of a certain disease i.e false negatives and false positives respectively.
The prevalence of disease in A is higher than in B meaning that assuming we don't know the specificity and sensitivity and assuming they are both the same whether in regards to groups A or B, A will have a higher PPV; in other words, those who test positive will truly have the disease is higher than in A than in B (this is what PPV is). Since B has a lower prevalence, NPV will be higher in B than A; meaning: Of those who test negative, more of those in group B than A will be truly negative when compared to a gold standard. PPV is usually increased by a higher specificity and a higher prevalence of disease, whereas NPV is increased by increased sensitivity and a lower prevalence of disease.
Hope this helps
i didnt even think of PPV nd NPV ...hop the same dosnt happen in exm as time limit increases the anxiety....neways thanx...
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  #9  
Old 06-04-2011
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Just donīt forget this graph!


Biostatistics #3 - Screening test-prev_posttest2.jpg
click image to enlarge
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  #10  
Old 06-06-2011
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awesome!!!!!!!!!!!!!!!!!
thanks!!!!!!!!!!!!!!!!!!!!!
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Biostatistics-Epidemiology, Figures-, Step-1-Questions

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