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Pair t test: defininng hypothesis

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  • #52841

    mand
    Member

    I am performing pair t test for my life from FEA and actual testing. my aim is to statistically check that my FEA predicts life within 10% of test life, so I am using pair-t test.
    The null hypothesis is
    Ho= difference in mean of test life and mean of fea life is 10%.
    Ha= difference in mean of test life and mean of fea life is <10%.
    pair-t test is giving p-value of 0.029 which states that I have to reject Ho and accept Ha.
    Now question is, Is my null hypothesis is correct? or what will be my null hypothesis based on problem statement.
    Please advice.
     

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    #186395

    bbusa
    Participant

    Since you are performing a paired t – test For Your Life , the p – value indicates there’s a 3% chance that you will outlive the mean life by 10%( whatever that is) but still think you won’t !Anyways , the paired t-test appears inappropriate for the case in question , a 2 sample t-test appears more appropriate – of course test for normality , equal variances etc.BBUSA

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    #186396

    bbusa
    Participant

    Forgot to mention your hypothesis statement is incomplete ( null hyp is incorrect ?)

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    #186397

    mand
    Member

    Than what can be my null hypothesis statement? Do I alway say that Ho= difference in mean life of test and mean life of fea is ‘0’ (zero)

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    #186398

    bbusa
    Participant

    Your hyp statement could be ( For verifying if the test life is 10% lesser than the mean life )H0 : mu1 – mu2 < 10% ( whatever the 10% means)
    Ha : mu1 – mu2 >= 10%Establish your hyp. with a 2 sample t – testBBUSA

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    #186407

    StuW
    Member

    SAM:
    Here is the general guideline for a paired t-test versus a 2-sample test.  If the measurments are made on the same experimental units, and those units are expected to differ from one another in a material way, then the paired t-test is appropriate, as it removes a source of variation (that due to the units themselves) and allows the test to clearly focus on the treatment of interest.  It sounds as if you are comparing predicted life against actual life, but what units are being measured?
    I can’t tell from your original description whether this should be a paired t-test or a 2-sample test.  I’m also not certain if you need to convert to a percentage metric, or not.   Is it necessary to convert each measurement to a percentage?   Just so you are aware, this is not a requirement of running the paired or 2-sample tests, but it might be appropriate if the data covers a large range. 

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    #186489

    Klerx
    Participant

    SAM:
    You can NOT, I repeat, you can not use a t-test, not 2-sample nor paired.
    Your conclusions are simply WRONG.
    Life is Weibull distributed and the t-test uses the Normal distribution. So you have to calculate confidence intervals in order to be able to do hypothesis testing.

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