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Failure Rate Question

  1. Sep 8, 2006 #1
    I am currently studying for a business qualification I am stubling over a question.

    It concerns calculating failure rate. The equation I know for this needs the number of failures over the total time.

    However the question is: "A device has a reliability of 0.8 over 2000 hours, calculate the failure rate."

    How can one firstly work of the number of failures from this? It doesn't give any other information, ie total sample size etc. just the reliability over 2000 hours.

  2. jcsd
  3. Sep 8, 2006 #2


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    Staff: Mentor

    I'm pretty sure 0.8 is the number of failures. To me it seems like the question gives the answer, but hen I guess I'm not sure what they are looking for. Wik has some info: http://en.wikipedia.org/wiki/Failure_rate
    So the population size would be 1, time is 2000 hours, and number of failures is .8.
    Last edited: Sep 8, 2006
  4. Sep 9, 2006 #3


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    Staff Emeritus
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    I believe reliability means 'rate of success', i.e. it works reliably 0.8 or 80% of the time. So 1 is all the time, and 1-0.8=0.2 would be unreliability, which is related to the failure rate.

    Look on the page for MIL-HDBK-217F
    Reliability Prediction of Electronic Equipment- Notice F

    One can download the pdf file.

    Weibull.com is a good resource for statistics and statistical analysis.
    Last edited: Sep 9, 2006
  5. Sep 11, 2006 #4
    Just to make this simple for you place it in your math formula like it says.

    0.8/2000 0.8 over 2000 hours. The answer of the efficiency is very low, but it's the truth according to the math. .0004 per hour is the answer according to the information furnished. Gee, slave labor does better than an efficiency rating like that, and it sounds like someone needs to pay someone better wages to get the job done. The machine is malfunctioning - tell whoever asked the question to fix the machine or up their wages to fair.

    For those of you who do not recognize significant numbers, in its stated form this is not a percentage. If someone meant it that way, their distinction is incorrect.

    What is the reliabillaty per hour is what I think the equation boils down to.

    Then stated as a failure rate in 20,000 attempts you only get 8 successes... that being the case expressed as a percentage, failure rate is 20,000/8 which is 2500% failure rate.
    Last edited: Sep 11, 2006
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