I'm trying to work through an example out of Goos and Jones (Ch 3), where they've set up a DoE screen and model of select two-way ineractions. All factors except "Block" are continuous, and the factors are coded as -1 and +1 for the two levels tested. When fitting the model, I'm confused that the Parameter Estimates pulldown gives each interaction term not as e.g. EtOH*n-PrOH, but rather as (EtOH + 0.1)*(n-PrOH-0.1) or (EtOH + 0.1)*Time (see IMG1.jpg). I'm not sure how to interpret this. Can anyone explain when and why JMP adds/subtracts a constant from each factor? It doesn't seem to always do this - I have another model where it does what I expect and gives the term as e.g. n-PrOH*EtOH (IMG2.jpg). Thanks!
It looks as though the midpoint of the range is being subtracted from those particular model terms. I do not see this issue when I ran through the same example. Some more information is likely needed to know exactly why this is happening.
What version of JMP are you using? The design in chapter 3 is not originally in terms of -1 and +1. Did you type all of the results in? Have you verified that there were no typographical errors? Do you have the coding property turned on? Is the "center polynomials" on the Fit Model dialog checked? Perhaps you could share your JMP data table so we can see EXACTLY what you see.
Hi Dan,
Thank you for your reply! I'm using JMP 15.0.0.
You wrote "The design in chapter 3 is not originally in terms of -1 and +1." That's true, but it seems that when reporting the factor-effect estimates i.e. in Tables 3.6 and 3.7, the estimates are reported for coded +1/-1 factor values. So I'd added columns of +1's and -1's in an attempt to replicate the analysis.
Attached is the JMP table file - "Ch03 - Adding Runs to Screening Expt." If the "Model - Table 3.7" script is run, you can see that in Parameter Estimates the EtOH*n-PrOH interaction term is given as "(n-PrOH-0.1)*(EtOH+0.1)" with an estimated value of -1.872.
You mentioned coding - I hadn't realized this was a column option. It turns out that when I let JMP know that those are coded columns (as in file "Ch03 - Adding Runs to Screening Expt - Coded"), it reports the interaction term as I expected, as "EtOH*n-PrOH," and also with estimated value of -1.872. So the parameter value is the same regardless.
But I'd still like to know what JMP sees differently that the +/- 0.1 is in there when the columns aren't coded, and what it means? In neither case does the Effects Summary table show the +/- 0.1, it's just in the Parameter Estimates.
Thanks again!
Jason
