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#1
07-04-2015
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 118 Threads: 10 Thanked 94 Times in 44 Posts Reputation: 104
Biostatistics questions

Hi guys,

Although in residency, I feel me addicted to this forum. Few interesting biostat questions I was asked by one of my attending.

A randomized study was conducted to evaluate the usefulness of aspirin for primary prevention of myocardial infarction (MI). There was a 44% relative reduction of MI in men treated with aspirin 0.56; 95 %( CI, 0.45-0.70) during the study period. The MI rates were 255/100,000 in the aspirin arm and 440/100,000 in the placebo arm.
About how many men will need to receive aspirin to prevent one MI in this study period?
A. 50
B. 550
C. 1000
D. 250
E. 189

Assuming there are 100 patients with coronary artery disease (CAD) involving a single vessel, 50 with CAD involving 2 vessels, 30 patients with CAD involving triple vessels, and 200 pts without CAD. The probability of a positive treadmill test in pts with single vessel disease is 0.4, the probability of a positive treadmill test in patients with double vessel disease is 0.6, and that of patients with triple disease is 0.8. Among patients without CAD, the probability of abnormal treadmill test is 0.1. What is the true positive rate when treadmill test is used to detect CAD?
A. 0.66
B. 0.52
C. 0.33
D. 0.72
E. 0.60
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#2
07-04-2015
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 463 Times in 298 Posts Reputation: 473

Quote:
B and B
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245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad
#3
07-04-2015
 USMLE Forums Scout Steps History: Not yet Posts: 96 Threads: 15 Thanked 19 Times in 18 Posts Reputation: 29

can u explain ur answer fr the second quest plz
#4
07-04-2015
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 463 Times in 298 Posts Reputation: 473

Quote:
 Originally Posted by pathophysio1 can u explain ur answer fr the second quest plz
ok
im not sure whether it is right or not
but here is how i derived that answer

from the 100 pts with one vessel disease probability of being tested positive would be 100x0.4=40, and for others 50x0.6=30 and 30x0.8=24 respectively
u sum up all this probabilities=94
then u devide 94 (sum of all probabilities) by total number of diseased individuals and receive the true positives (those who test positive from a diseased people) 94/180
__________________
Everything is possible for him who believes (MARK 9:23)
245/247/passed on 1st attempt/223/2mos Obsie /3 US LORS/visa not needed/2008 grad

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