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#1
03-09-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 529 Threads: 57 Thanked 284 Times in 152 Posts Reputation: 294
probability question!

A serological test for detecting prostate cancer is negative in 95% of the cases. If the test is used with 8 consecutive blood samples taken from patients without the disease, what is the probability of getting at least one positive test result?
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

#2
03-09-2012
 Banned Steps History: Not yet Posts: 176 Threads: 34 Thanked 117 Times in 69 Posts Reputation: 127

A serological test for detecting prostate cancer is negative in 95% of the cases. If the test is used with 8 consecutive blood samples taken from patients without the disease, what is the probability of getting at least one positive test result?
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

Explanation
Test is Truly Negative in 95% of the cases, and Falsely Positive in 5%. The scenario states that the test is used with 8 blood samples. Each testing should be assumed to be an independent event. Than mean that each time the test is performed in 1 blood sample there is 0.95 (95%) probabilities that the test will give a True Negative result and 0.05 (5%) probabilities that the test will give a False Positive result.
 The above post was thanked by: MzLizMD (03-09-2012)
#3
03-09-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 529 Threads: 57 Thanked 284 Times in 152 Posts Reputation: 294
why not c?

why is it not c?? ughhhhh i was never good with the exponents stuff

#4
03-09-2012
 USMLE Forums Guru Steps History: Not yet Posts: 414 Threads: 30 Thanked 87 Times in 70 Posts Reputation: 97
.05*8

Quote:
 Originally Posted by bunny A serological test for detecting prostate cancer is negative in 95% of the cases. If the test is used with 8 consecutive blood samples taken from patients without the disease, what is the probability of getting at least one positive test result? 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 Explanation Test is Truly Negative in 95% of the cases, and Falsely Positive in 5%. The scenario states that the test is used with 8 blood samples. Each testing should be assumed to be an independent event. Than mean that each time the test is performed in 1 blood sample there is 0.95 (95%) probabilities that the test will give a True Negative result and 0.05 (5%) probabilities that the test will give a False Positive result.
isn't the question asking false positive ?
which is .05 per patient and to get value for all
.05*8??????
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#5
03-25-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 529 Threads: 57 Thanked 284 Times in 152 Posts Reputation: 294
anyone??

can any1 explain this please?? I think the answer is C. i dont understand why it is E ... UW says its E .. i just dont get it!
#6
03-25-2012
 USMLE Forums Addict Steps History: 1 + CS Posts: 193 Threads: 29 Thanked 134 Times in 71 Posts Reputation: 144

all i know is this --> C & E are the same!!

C. 0.05 raised to the power 8
E. 1-0.95 raised to the power 8

explanation? because 1 -0.95 = .05

i sure do know my math :P
#7
03-25-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 529 Threads: 57 Thanked 284 Times in 152 Posts Reputation: 294

Quote:
 Originally Posted by drhma all i know is this --> C & E are the same!! C. 0.05 raised to the power 8 E. 1-0.95 raised to the power 8 explanation? because 1 -0.95 = .05 i sure do know my math :P
lol no let me re-write that one. option E. is 1-(0.95raised to the power 8!) .. C & E are different! my bad, i didnt write the options clearly!

 Tags Biostatistics-Epidemiology, Step-1-Questions

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