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#1
11-05-2014
 USMLE Forums Newbie Posts: 7 Threads: 3 Thanked 1 Time in 1 Post Reputation: 11
USLME step1 question

A new serologic test for detecting prostate cancer is negative in 95% of patients who do not have the disease. If the test is used on 8 blood samples taken from patients without prostate cancer, what is the probability of getting at least 1 postive test result?
- can someone explains this to me! Thank you!

#2
11-05-2014
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 252 Threads: 18 Thanked 129 Times in 98 Posts Reputation: 139

Quote:
 Originally Posted by jyhhsieh A new serologic test for detecting prostate cancer is negative in 95% of patients who do not have the disease. If the test is used on 8 blood samples taken from patients without prostate cancer, what is the probability of getting at least 1 postive test result? Why answer is 1-0.95^ 8 instead of 0.95^7 * 0.05 which is not the answer choice - can someone explains this to me! Thank you!
It's just a question of probability. 0.95^8 gives you the probability of having a negative result in the 8 blood samples. 1-0.95^8 gives you the probability of a positive result.

Why did you multiply it by 0.05 in the first place? and take a power to the 7?
 The above post was thanked by: jyhhsieh (11-05-2014)
#3
11-05-2014
 USMLE Forums Newbie Posts: 7 Threads: 3 Thanked 1 Time in 1 Post Reputation: 11

becuz it asked at least one positive so shouldn't it be 0.05 as a positive result with 7 time of negative so 0.05 * 0.95 power to 7!!! This is just confuse heck out of me twice for my 2nd time on Uworld!

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