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#1
02-10-2014
 USMLE Forums Guru Steps History: Not yet Posts: 394 Threads: 128 Thanked 109 Times in 89 Posts Reputation: 119
calculate probability

Assume that any single single laboratory test in a 3 test battery obtained from healthy volunaers having 5% chance of falling outside the normal range.Which of the following is closest probability that healthy volunteer has same result on all test????

5
40
85
95
100

#2
02-11-2014
 USMLE Forums Scout Steps History: Not yet Posts: 13 Threads: 2 Thanked 6 Times in 4 Posts

Quote:
 Originally Posted by medicalbiology Assume that any single single laboratory test in a 3 test battery obtained from healthy volunaers having 5% chance of falling outside the normal range.Which of the following is closest probability that healthy volunteer has same result on all test???? 5 40 85 95 100
I think you mean: the probability that healthy volunteer will have "normal" result for all 3 tests..
which is 95%x95%x95% = 85%
#3
02-12-2014
 USMLE Forums Scout Steps History: Not yet Posts: 25 Threads: 4 Thanked 4 Times in 3 Posts Reputation: 14

Quote:
 Originally Posted by whitecoat I think you mean: the probability that healthy volunteer will have "normal" result for all 3 tests.. which is 95%x95%x95% = 85%
How did you do that?can you explain please

#4
02-12-2014
 USMLE Forums Scout Steps History: Not yet Posts: 13 Threads: 2 Thanked 6 Times in 4 Posts

Quote:
 Originally Posted by arvine_14 How did you do that?can you explain please
In this question, the test results are independent events
(i.e. the result of one test tells you nothing about the other 2 results) and the rule is we combine probabilities for independent events by multiplication.

The chance of "having the first test normal" equals 95%, which also equals the chance of having the second test normal as well as the third.

So the chance for having them all normal = the chance of having the first one normal x the chance of having the second one normal x the chance of having the third one normal.
 The above post was thanked by: arvine_14 (02-12-2014)
#5
02-12-2014
 USMLE Forums Scout Steps History: Not yet Posts: 25 Threads: 4 Thanked 4 Times in 3 Posts Reputation: 14

Quote:
 Originally Posted by whitecoat In this question, the test results are independent events (i.e. the result of one test tells you nothing about the other 2 results) and the rule is we combine probabilities for independent events by multiplication. The chance of "having the first test normal" equals 95%, which also equals the chance of having the second test normal as well as the third. So the chance for having them all normal = the chance of having the first one normal x the chance of having the second one normal x the chance of having the third one normal.
Thanks a lot whitecoat. It was a crystal clear explanation and quite simple too. I forgot how to solve this type of questions actually. But thanks anyways. :-)

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