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#1
08-07-2012
 USMLE Forums Addict Steps History: Not yet Posts: 109 Threads: 24 Thanked 19 Times in 11 Posts Reputation: 29
probability question

A new serological test for detesting prostate cancer is negstive in 95% of patients who do not hv d disease.If d test is done on 8 blood samples from patients wtout prostate cancer, wat is d probability of gettn atleast 1 positive test result??

#2
08-08-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

Quote:
 Originally Posted by sruthi A new serological test for detesting prostate cancer is negstive in 95% of patients who do not hv d disease.If d test is done on 8 blood samples from patients wtout prostate cancer, wat is d probability of gettn atleast 1 positive test result??
Specificity of a test is 95%, so a chance of a false positive result is 5%.
The probability of getting at least 1 positive test result in pts who don't have the disease is max 5% (bc e.g. if ask what is the chance of getting two false positive results it will be .25%)
#3
08-08-2012
 USMLE Forums Addict Steps History: Not yet Posts: 109 Threads: 24 Thanked 19 Times in 11 Posts Reputation: 29

the options were;

A. 1-0.05*8
B. 0.05*8
C. 0.05 raised to the power 8
D. 0.95 raised to the power 8
E. 1-(0.95 raised to the power 8)

the answr given ws E...

explanatn ws if each of d 8 tests were taken as an independent event, d probability to get a negative always will b 0.95 multiplied 8 times..since d total probability will always b 1, d probability to get 1 positive will 1-(0.95 raised to 8)...
i didnt get d last part!!
n also y cant it b C??

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