Adding Center Points to a Main Effects DOE with Factor Constraints

[#1]

Hello,

I am designing an 7 factor DOE with a constraint on placed on two of the factors. I would like to add center points to the experiments, but when I select "Define Factor Constraints" the option to add center points disappears.

Are these two options exclusive of each other? Before I attempt to add the constraints I can freely add center points.

Thanks,

David

[#2]

I cannot replicate this problem. What version of JMP are you using? Are you using a Custom Design? What is the nature of your factors? Are they all continuous? What is the linear constraint? Does that linear constraint not allow center points? More details, please.

[#3]

Hi Dan-

Sorry for incomplete info - to answer your questions:

- JMP 16

- Custom Design

- 6 continuous factors and 1 discrete numeric

- The nature of the the constraint is such that when “f” is greater than 20, “g” cannot be 13. I used the "Disallowed Combinations Filter" to make the constraint but the behavior is the same regardless of filter type.

To make it easier I took some screenshots and put them on slides here:

Google Slides Presentation 

When I do not have a constraint I can add center points without a problem.

Thanks,

David

[#4 · Accepted solution]

Thanks for all of the details! I can certainly see the issue now.

One thing that I noticed is that when I create the design, if I select a Linear Constraint, I still have the option for center points. As soon as I select Disallowed Combinations, the center points will disappear. If I switch to linear constraints, it does not appear! Putting the constraints to none, center points reappear, then choose linear constraints and it will stay.

I can see that this behavior has nothing to do with what your constraint is. I am not sure why, so for the best answer you may wish to contact JMP support. I do believe that this behavior is intended though. My best guess is that once you start adding certain combinations that are not possible, determining what is the "center" might be difficult. This is just a guess on my part. Perhaps tech support can provide a better answer.

[#5]

Thanks - will give support a try.

[#6 · Accepted solution]

Hi David. I echo what Dan stated - the behavior you described is expected. In Custom Design, the option to add center points disappears when selecting a factor constraints option, because with a restricted design space it is difficult to ensure that the center point lies in the allowable region. 

If you want to add center points in a design with restrictions on the design space, a workaround is to add rows manually in the table after creating the design.