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MTBF

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  • #28941

    Ovidiu Contras
    Participant

    Please help me with this one :
    Data : 15 machines tested
             : machines had the same design
             : machines not tested in the same time ,but over a longer period of time
             : each machine ran 500h without failure
    Question : does this mean that we proved an MTBF of 2503h ,with 95% confidence?
    Your help is much appreciated ,as usual.

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    #72982

    RR Kunes
    Member

    Remeber that statistically you only have a 38% chance of meeting calculated MTBF. Therefore if you have a goal of 2500 hours you must demonstrate 6579 hoours.
     

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    #72983

    Ovidiu Contras
    Participant

    Thanks for the reply ,and please forgive my ignorance (I am not a reliability expert) :
    from where the 38% comes from ?

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    #72984

    Al
    Participant

    I think the recognised way of doing this is involves the chi-squared function. 
    I do not have details to hand, but any textbook with ‘Reliability’ in the title will have it.

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    #72991

    Al
    Participant
    #72995

    A Friend
    Participant

    You are correct IF you are willing to assume the failures follow an exponential distribution.
    If so, the lower 95% confidence interval for the Mean Time To Failure (which for exponential is the same thing as MTBF) is 2503.56 hours, according to MINITAB’s Parametric Distribution Analysis – Right Censoring.  This means that there is a 95% probability (confidence) that the actual, but unknown, MTTF is above 2503.56 hours.
    This result from MINITAB matches the recommendations from Meeker & Escobar’s “Statistical Methods for Reliability Data” from Wiley. 
    From an exponential perspective, you have already tested 15×500=7500 hours without a failure.

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    #72997

    Ovidiu Contras
    Participant

    Thank you very much for your responses .It is very much appreciated .

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