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Homework Help: Probability Problem

  1. May 20, 2014 #1
    1. The problem statement, all variables and given/known data

    Suppose that upon using a particular testing device, a defective component has a 70% chance of being found as defective, and a non-defective component has just a 10% chance of being found defective. Suppose also that within a quality control batch, the probability of a randomly selected component being defective is 12%.

    i)What is the probability that a component selected at random from the batch is found to test positive (i.e. tests as being defective)?

    ii)What is the probability that a randomly selected component is defective, given it is found to test positive?

    2. Relevant equations

    3. The attempt at a solution

    i)I said:

    (0.12)(0.7) +(0.88)(.1)=0.172

    ii)'m having trouble wrapping my head around this one. Did I use the correct method for the first part? Also any help in the second part would be much appreciated.
  2. jcsd
  3. May 20, 2014 #2


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    Homework Helper

    This problem is a simple application of Bayes' Theorem. Are you familiar with it?

    The answer to the first part is correct, by the way, although it's better to express the probability as a percentage since you're given percentages to begin with.

    Before applying Bayes' Theorem, start defining your events and your conditional probabilities, e.g. ##p(D^+)## is probability of a random article being defective and ##p(T^+|D^+)## is probability of test positive IF article is defective. Surely, you've seen this sort of notation?
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