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Model reduction

SaraA

Hello,

According to the JMP learning model for DOE analysis, non-significant terms in the model can be removed, starting with the non-significant higher order interactions (= model reduction).

However, is it really that simple? Are there other things to take into considerations before removing non-significant terms? Does removing a non-significant term necessarily meant that this factor has no/minor effect on your output/response variable?

Additionally, does model reduction also apply for REML/mixed models?

Thank you for any clarification on this matter.

Sara

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P_Bartell

It's not that simple. Removing model terms or not ranks right up there with the ongoing discussion of use/abuse/misuse of p-values. In fact, to me, it's kind of two sides of the same coin.

Having said this, however, I'm safe in saying removing a model term does not mean a factor(s) involved in the parameter estimate has '...no/minor effect on your output/response variable?' It just means over THAT particular inference space you are deciding to not include the term in the model. That decision is worlds away from declaring to God and all who will listen there is 'no /minor effect.' Maybe the levels you choose (if modeling a designed experiment) were too narrow for a signal to rise above the noise. Maybe the noise(s) in the inference space are overwhelming the effect of the factor(s). And don't get me started on 'p-value' interpretation...all terms are 'significant' at some p-value. Who picks that significance value? By what criteria?

And IMO same can be said for just about any modeling method (REML/mixed and others) I can think of.

SaraA

Hi P_Bartell,

I understand the complexity of that matter. However, my reasoning is that at some point you want to be able to make inferences based on the results of the DOE you have performed. Otherwise, what is the purpose of performing it... So from a practical point of view, what do you do with the non-significant terms?

Thank you

Sara

statman

Pete is, of course, correct and wise in his advice. Particularly the issue of inference space. When you ran your experiment, how representative was the noise in the study? How was noise handled in the study? (e.g., did you restrict, block, repeat. replicate, partition with split-plots)? Is the estimate if MSE reasonable and does it represent the true random error? If not is not, then statistical significance is meaningless. Remember, statistical significance is a conditional statement!

When doing model reduction, here are things you should consider:

1. Practical significance. This has nothing to do with statistical significance which is a comparison of effects to MSE. Do the factors/interactions have a practical significance? Another way to say this is you should evaluate the model in terms of scientific or engineering merit. Are there rational and logical hypotheses that support the identification of interesting and not interesting terms?

2. R-square-R-square adjusted delta. As you remove uninteresting term from model, the delta should diminish. Also the size of the R-square adjusted.

3. Residuals. How do they look? Are there any outliers, unusual patterns, etc. as you remove terms?

4. p-values. These might be useful for the first cut, but they are less useful (or useless) as you iterate. What happens to the insignificant mean squares? They are added to the MSE WITH the DF's essentially decreasing the MSE and increasing the F-values in a biased fashion.

5. Useful. The model must be useful. The mission is not to create the most complex model you can get, but to create a model the is predictive and useful.

Of course, one of the advantages of JMP is the ability to quickly change the model and re-evaluate.

P_Bartell

In answer to your question, "Otherwise, what is the purpose of performing it...". By 'it' I assume a designed experiment...I'll answer that from my admittedly somewhat biased frame of reference coming from industry as opposed to a research environment.

Far and away the primary purpose for conducting our designed experiments was not to '...make inferences...'. But to solve a practical problem. I'm not saying searching for 'inferences', which I'll define as new novel knowledge, isn't a worthy endeavor...it's just not how we found ourselves. So what is the ultimate purpose for conducting these designed experiments? If it's to solve a practical problem...then I'd try it both ways...modeling with and without the 'insignificant' terms and view the utility of the model in the context of solving the practical problem at hand. If it's new knowledge, then I'd consider additional experimentation before making blanket statements like 'no/minimal effect' to expand the inference space under different noises (suppliers, equipment, operators, scalability, and on and on) and nuisance variables and other issues, like factor level spacing, measurement system noise, ...and rarely is this accomplished with one experiment. Where I worked, we called this, ' understanding representation risk'.

A simple purely illustrative example, but situation we encountered more often than any of us would have liked. We would run a DOE on pilot equipment and make an inference 'that coating speed' has no effect on the response of interest. But then we scale up to production equipment...and all of a sudden 'coating speed' does have an effect. How can this be? New process...new noises. It's about how 'representative' the experiment is to future conditions. Deming wrote at length about the issues associated with enumerative vs. analytic studies.

Enumerative vs. Analytic Studies

Only by understanding this wide landscape can one confidently approach making blanket statements like, 'no/minimal effect'.

SaraA

Considering these responses, I would argue that the JMP DOE learning module oversimplifies the analysis of a DOE.