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#1




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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aknz (10082011), Amenah (10082011), dinosaur108 (10082011), excellence (05182014), nevillenic (06102011), NGaleas (06112011), praru (06232016), rulz (10082011), struggle (06102011), sumedha (10082011) 
#2




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 (2019) and 98 false positives (980882) 
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bebix (06102011), dinosaur108 (10082011), dr z (06142011), excellence (05182014), nevillenic (06102011), struggle (06102011), sumedha (10082011) 
#3




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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struggle (06102011) 
#5




just give it a try...btw, apx85īs answers are correct

#7




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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bebix (06112011) 
#8




Great and i finally get it.. took me a day to review and understand... i hate this type of question... THANKS bebix!
Answers: 1. 9819 = 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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bebix (10082011) 
#9




Quote:

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dinosaur108 (10082011), rulz (10082011) 
#10




Thanks bibex,great work

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BiostatisticsEpidemiology, Step1Questions 
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