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How to find probability in repeated trials?

  1. Jun 27, 2013 #1
    Info: I made the mistake of starting engineering grad school years after finishing my undergrad, and forgot my basic prob/stat. This is related to reliability engineering.

    1. The problem statement, all variables and given/known data

    How many arbitrarily selected units do I have to test so that my probability is 0.85 of finding one at least one defective product if I have a 0.05 defect rate for this particular product.

    2. Relevant equations

    Binomial Probability Formula

    binomial%20probability%20formula.gif

    3. The attempt at a solution

    I tried setting the formula to be equal to 0.85, k=1 (because I only need one to fail), and left number of trials as "n" to solve for. I have tried to solve for this and was having a tough time, and I am not even sure if this is the correct approach.

    Edit: p=0.05 and q=1-p=0.95
     
    Last edited: Jun 27, 2013
  2. jcsd
  3. Jun 27, 2013 #2

    Dick

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    The probability of getting at least one failure after k trials is one minus the probability of getting no failures after k trials. You should find the 'no failures' case easier to solve.
     
  4. Jun 27, 2013 #3

    haruspex

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    What Dick said. To elaborate, setting k=1 gives the probability of exactly one failure - you want at least one.
     
  5. Jun 27, 2013 #4

    haruspex

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    What Dick said. To elaborate, setting k=1 gives the probability of exactly one failure - you want at least one.
     
  6. Jun 27, 2013 #5
    Thanks for the help guys. That was a herp-derp moment for me.
     
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