Analysis of Plackett-Burman designHi all,
I have run a Plackett-Burman foldover screening DOE using 9 continuous factors. I included 3 center points to detect curvature.
I am unsure if for the analysis of the data table I should run the 'Screening' script or use the fit model platform. If I use the fit model platform, should I only include the main effects to construct the model effects since a PB design only allows estimation of main effects? When doing this, I have a significant lack of fit test and the model it does not detect curvature. Whilst when I use the screening script the lack of fit test is non-significant and I see that for some factors, the relationship with the reponse is non-linear.
Thank you for your help.
Sara
Design of ExperimentsRe: Analysis of Plackett-Burman designThe P-B designs can potentially estimate more than the main effects. They might produce correlated estimates for some terms, but they generally avoid confounding by using regular fractional factorial designs.
The Screening platform is a bridge to the Fit Least Squares platform. Its sole purpose is term selection before estimation.
The lack of fit is presumably because the replicated runs (center points) exhibit less variance than the model. Select the center points. Please give them a unique color (not black) and exclude (not hide) them. Run the model in Fit Least Squares again. You won't have a Lack of Fit test, but you can see the center points in the Actual by Predicted plot at the top. Are they out of line with the rest of the runs?
Re: Analysis of Plackett-Burman designI also do not understand why this Screening script appears for this PB design while it was not present for a main effects screening design that I performed a few months ago.
