Questions about the factor value range in Custom DOE

Question

Hi~ I'm engineer to develope a antenna.

I want to prediect the antenna performance by manufacturing tolerance using the DOE.

Manufacturing tolerance vary by vendor. (ex. A vendor tol. : +/-0.004, B vendor tol. : +/-0.016)
So I defined the range of factor values widely so that they can contain all the vendor's tolerance value.
I selected the range of each vendor's tolerance using "Nomal truncated" in pofiler of Fit model and checked the response(VSWR)'s SD value.
When the range of factor's value varies as in above case, is it correct to simulate with "normal truncated"?
I made two designs to verify it and compared to standard deviation(SD) values of responese(VSWR).
In case1, range of factor(ppg_tol) is +/-0.004 and it of case2 is +/-0.016.
In profiler of fit model, SD of case1 is 0.09 and SD of case2 is 0.05.

I want to know why the two SD values are different.
(I used custom design and both designs have the same condirions except ppg_tol)

And How is the best DOE design method to compare various factor's ranges?

Thanks

Mark_Bailey · Answer

I still do not know much about what you are studying but let me suggest an approach for you to consider.

I assume that your response is continuous. I further assume that you have specifications for this response. So I would design your experiment using factor ranges as wide as possible. Your analysis (after confirming that the model has no bias) with the simulation should provide you with the failure probability.

The simulation depends on the model and the model depends on the data. A good model should accurately predict good and bad responses. The factor levels (ranges) should never be limited by expectation of a good response level.

mintkoko77 · Answer

Thank you for your reply.

You're right that the two models are differ. I made two designs for to verify my methodology.

I will change my question.

I want to predict a failure rate of two product.The difference between the two products is a range of factor value.(The factor is ppg_tol.)

ppg_tol of 'A' product is +/-0.004 and it of 'B' product is +/-0.016.

In this case, how should I design custom DOE? Do I have to make two DOE designs?

I made only the DOE design for 'B' product and predicted a result of 'A' product using 'Normal truncate' of simulator.

Is it wrong to compare SD of both products in this way?

Mark_Bailey · Answer

The profiles shown in the two cases are different. It seems that the terms in the model and the parameter estimates for case 1 are not the same as for case 2. Such differences could easily account for the difference in the simulation results.

Also, case 2 appears to have fewer terms, so the RMSE is higher. You can see that the confidence interval on the profiles is different for the two cases. That difference would also change the simulation results.

I think that most of the reason for the difference is the change in the model between case 1 and case 2, though.