third order terms in linear model

Michal

Dear JMP Experts,

I would like to hear your opinions on the topics below:

1)Does it make sense to include 3rd order (cubic, interaction) terms into a model with experimental runs consisting of factors investigated on 3 levels only?

2)Under what circumstances would you include 3rd order terms that in respect to industrial process model?

Thanks very much,

Michal

Mark_Bailey

First, you cannot estimate the parameters for the third-order terms with only three levels. If you have an experiment that successfully fit the second-order model albeit with lack of fit, then use DOE > Augment Design to add the new terms and make the new runs that compliment the original runs.

 

Second, I would explore the third-order terms if either my second order model exhibited significant lack of fit or I was unable to verify the second-order model predictions. This failure indicates model bias that cannot be tested as lack of fit.

 

The second-order model successfully models most cases with a non-linear response but over an extended range, either a higher-order linear model or another model entirely (e.g., neural network) might be required for elimination of the bias. For example, if your response exhibits an asymptote, the polynomial function will not successfully model the response.