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DOE error DF = 0

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  • #48766

    REL
    Participant

    I am trying to analyze a 2-factor, 3-level full factorial experiment in Minitab but I keep getting a “Denominator of F-test is zero” message. It also shows that the DF for error is zero. I do not know how this can be and I am at a loss as to how to proceed. I’ve tried multiplying my data by 10 but this does not help. It seems as though no matter what I do to the data I keep getting the same message. Any assistance would be much appreciated.

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    #165325

    Robert Butler
    Participant

    If you have a 2 factor 3 level design you will have 9 experiments.  If you have told your program you want to test for all of the factors this design will allow this means you are asking the machine to test for
    A, B, A*A, B*B, A*B, A*A*B, A*B*B, A*A*B*B and the mean (intercept) – this is 9 terms and you have only 9 data points – you have no degrees of freedom left for error. 
     If you can’t replicate at least one point in your design you will have to ask your program to run a forward selection with replacement stepwise routine and not a backward elimination in order to come up with a model.

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    #165328

    Van Kim Ban
    Member

    First  try  to understand  the  DoE concept before  using  the  minitab. 

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    #165640

    Jen
    Participant

    REL,
    Your DOE design is saturated, you take up all your degrees of freedom with the factors and leave none for the error factor in the ANOVA.  You can do a couple of things.  First, if you have the resources you can run the experiment again.  You will then have a replicate to provide some information for the error variable.  Or if any of your settings are the typical settings of what you are analyzing you could turn them into centerpoints and run your analysis with center points.  This will give you p-values to analyze.  If the centerpoints show as not being significant you can reduce them from the analysis and continue until you determine if the interaction, a factor, or all are significant.  I hope this helps you.

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    #165643

    Jen
    Participant

    REL,
    Keep in mind what I put in my previous posting will have you reanalyze the data collected as a 2k with centerpoints, you will not have a general full factorial anymore.

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