Biostatistics #19 - Screening tests (part b) - USMLE Forums
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Old 06-10-2011
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Stats Biostatistics #19 - Screening tests (part b)

Your screening facility can process 1000 people per week. Assume you are attempting the early detection of a disease with a prevalence of 2% and that your test has a sensitivity of 95% and a specificity of 90%.

1.- How many of the people screened in a week will test positive?

a. 19
b. 98
c. 103
d. 117
e. 200

2. Of these, in how many will the results be truly positive? and in how many will they be falsely positive?
a. 19
b. 98
c. 103
d. 117
e. 200

3. Of these, in how many will they be falsely positive?
a. 19
b. 98
c. 103
d. 117
e. 200

4.- Since a patient tested positive, what is the probability that he has the disease?
a. <0.01%
b. 2%
c. 10%
d. 16%
e. 84%
f. 95%
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Old 06-10-2011
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1. D, 117
2. A, 19
3. B, 98
4. D, 16%

Since prevalence is 2% out of 1000 people per week, 20 people will have disease and 980 will not.

With sensitivity of 95%, 19 people with disease will screen positive (20*.95)
With specificity of 90%, 882 people without the disease will screen negative (980*.90)

This means that there will be 1 false negative (20-19) and 98 false positives (980-882)
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Old 06-10-2011
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Took me a few minutes to get all the math straight but once I was able to come up with the 2X2 table it made the questions straight forward, assuming I got the table right!
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Old 06-10-2011
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I am trying to solve it..its hard though
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Quote:
Originally Posted by struggle View Post
I am trying to solve it..its hard though
just give it a try...btw, apx85īs answers are correct
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i m trying too
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Default Ans.

Great question to practice screening tests, using a different approach. Thanks.
1.D.117
2.A.19
3.B.98
4.16%

I agree with apx85, number 4 refers to the PPV, which is calculated with:
PPV= TP/TP+FP
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Old 10-08-2011
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Great and i finally get it.. took me a day to review and understand... i hate this type of question... THANKS bebix!





Answers: 1. 98-19 = 117 will be positive
Answer: 2. 19 will be TRUE prositive
Answer 3. 98 will be Falsely Positive
Answer 4. PPV will be 19/19+98 = 16%
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Quote:
Originally Posted by rulz View Post
Great and i finally get it.. took me a day to review and understand... i hate this type of question... THANKS bebix!

Answers: 1. 98-19 = 117 will be positive
Answer: 2. 19 will be TRUE prositive
Answer 3. 98 will be Falsely Positive
Answer 4. PPV will be 19/19+98 = 16%
Well done!!!!
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Old 10-08-2011
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Thanks bibex,great work
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