DOE – No Significant P Values

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    If all the pvalues not significant (greater than .05), can you still use an optimizer to determine the best settings? 


    Robert Butler

     If you run a DOE and have done a proper analysis (normalized the X’s, checked the residual plots for trending etc.) and you find that none of your variables are significant then your design is telling you that, over the ranges of the X variables you examined, the X variables have no significant effect on the responses that you measured.  In this instance there is no best setting.  The usual practice in such cases is to set the X’s at the levels that are most convenient.  If convenience is defined in terms of lowest cost then the most convenient settings would be those that minimized raw material cost.
     In those cases where I get a result like the one you describe I check the statistics of the stepwise regression on a step-by-step basis to make sure that the level of significance of each of the rejected variables is well below my cut point.  If I have terms that would be significant between 90 and 95%  I will open up the selection criteria and let the machine run the analysis again.  I will report that nothing met my predetermined level of significance and I will present the findings of the lower probabilites of success. I ask everyone concerned if the terms that entered the model under the relaxed selection conditions make physical sense and if the “extremes” in the Y responses that are predicted with such models are physically meaningful.  If the answer to both of these questions is affirmative we usually will test the “relaxed” model with further work. 



    Dear Ry,
    In addition to what Mr Butler has mentioned , no significant p values may also indicate that an important factor was missed out during DOE design.I have encountered a practical case to prove this point.Hence talk to the experts concerned to investigate more on this issue
    Girish S Dandekar

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