1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Probability Bayes formula Question

  1. Sep 26, 2009 #1
    Hi guys, i've got 2 probability questions that i couldn't solve. Please help.

    1) A computer program has an error that causes the program not to function perfectly. n programmer are assigned separately to detect the error. The probability that each programmer will detect the error is 0.875. Determine the value of n if the probability that at least one programmer detects the error is 0.998

    2) A new skin test is devised for detecting tuberculosis. To evaluate the test before it is put into use, a research indicates tuberculosis in 96% of those who have it and in 2% of those who do not. It is known that 8% of the population has tuberculosis.

    a) Find the probability of a randomly selected person having tuberculosis given that the skin test is positive.

    I've gotten this right, Its (Have tuberculosis|Test is positive) so what i did was using (0.96 x 0.08) + (0.02 x 0.92) the value for all test is positive. Then 0.96 x 0.08 divide by the value i just got, i got the answer right.

    b) Find the probability that a person has tuberculosis given that the test indicates no tuberculosis is present.

    What i had in mind is to reverse the probability for all test is positive and get all test is negative and do the same, but it wasn't right.

    c) Find the probability of the skin test giving a false positive result.

    Please help :) Thanks
  2. jcsd
  3. Sep 26, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You would do well to use symbols and equations. For part (a) if you let X = number of programmers finding the error then X is binomial with parameters n and p = .875. You are looking for P(X ≥ 1) which is 1 - P(X = 0) to be .998.

    For the second part, again, use some mathematical notation.

    Let P be the event the test is positive
    Let T be the event the patient has tuberculosis.

    In part (a) you were calculating P(T|P) using the (unwritten) Bayes formula. To make your solution readable you should write:

    [tex]P(T|P) = \frac {P(P|T)P(T)}{P(P|T)P(T) + P(P|\overline{T})P(\overline{T})} =[/tex]

    and put your numbers in after the = sign.

    For (b) you are asking for [tex]P(T|\overline{P})[/tex]. If you will begin by writing out the formula for that you may see how to calculate it. Using the formulas can make all the difference.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook