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Old 11-05-2014
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Default 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?
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!
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Old 11-05-2014
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Quote:
Originally Posted by jyhhsieh View Post
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?
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Old 11-05-2014
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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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