








USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam 

Thread Tools  Search this Thread  Display Modes 
#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% 
The above post was thanked by:  
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) 
The above post was thanked by:  
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!

The above post was thanked by:  
struggle (06102011) 


#5




just give it a try...btw, apx85´s 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 
The above post was thanked by:  
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%
__________________
Nothing is Impossible, the Word Itself says "I'm Possible" 
The above post was thanked by:  
bebix (10082011) 
#9




Quote:

The above post was thanked by:  
dinosaur108 (10082011), rulz (10082011) 
#10




Thanks bibex,great work

Tags 
BiostatisticsEpidemiology, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Biostatistics #18  Screening tests  bebix  USMLE Step 1 Forum  16  01122015 06:40 PM 
NBME tests!  ferdenand  USMLE Step 2 CK Forum  2  10162013 10:22 AM 
Biostatistics #14  Screening test  bebix  USMLE Step 1 Forum  6  06062011 07:38 PM 
Biostatistics #3  Screening test  bebix  USMLE Step 1 Forum  9  06062011 02:17 PM 
