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* Biostat q discussion
 #860854  
  selusmle3 - 12/27/19 19:14
 
  To protect blood supplies from contamination , screening all donors for heptitis C is reqiured .This screening has 95% sensitivity and 90 % specificity and is used on donors in which 10% are known to have heptitis C infection .Which one of the following is the best estimate of the chance of that donor who tests negative is actually free of the infection ?

A- 45%
B- 95%
C- 85 %
D-90 %
E - 95%
F- 99%
 
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* Re:Biostat q discussion
#3386364
  britfelo - 12/29/19 14:36
 
  what they are asking for is the negative predicted value. it is calculated by dividing True negative by true negative and false negative which is equal to: 810/ 810+50 = 810/860 = 95%  
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* Re:Biostat q discussion
#3386365
  britfelo - 12/29/19 14:42
 
  here is the table:


**********/ Disease*********** Healthy

Test positive: 95 ----------------- 90 ------- 185

Test Negative: 5 ---------------- 810 ------- 815

Total: 100 ----------------- 900 -------- 1000

Negative predictive value = 810/ 815 = 99%
It looks like it is F.
 
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* Re:Biostat q discussion
#3386366
  britfelo - 12/29/19 14:44
 
  Your other question the correct answer is A. >99%  
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* Re:Biostat q discussion
#3386367
  selusmle3 - 12/29/19 16:07
 
  It started with sample of 1,000 because it is a nice round number
10% of 1,000 = 100 infected
95% sensitivity of 100 infected
90% specificity 0f 900 non infected
2x2 table
NPV = TN/TN+FN=810/810+5=810/815 ~ 99%
 
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* Re:Biostat q discussion
#3386368
  selusmle3 - 12/29/19 16:22
 
  @ britfelo - yes it is F 99 %
thanks I got it now I prefer your approch .

the second q - A is not the suggested answer .
 
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* Re:Biostat q discussion
#3386369
  britfelo - 12/29/19 16:24
 
  You are welcome. what is the suggested answer for 2nd answer?  
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* Re:Biostat q discussion
#3386370
  selusmle3 - 12/29/19 16:39
 
  It is C 0.64 it says it is asking coefficient of determination= r square
r =0.8
r square = 0.8 x 0.8 =0.64

64% of variability in plasma homocysteine level is explained by folic acid intake
 
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* Re:Biostat q discussion
#3386375
  tunoe - 12/29/19 23:45
 
  thanks  
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