Mixture Design Tutorial (Part 1 – The Basics) - Statease.info

For each set of terms, the probability (“Prob > F”) should be examined to see if it fallsbelow 0.05 (or whatever statistical significance level you choose). Adding terms up toquadratic will significantly improve this particular model, but when you get to thespecial cubic level, there’s no further improvement. The program automaticallyunderlines at least one “Suggested” model. Always confirm this suggestion byreviewing all the tables under Fit Summary.The lack of fit table compares the residual error to the pure error from replication. If theresidual error significantly exceeds the pure error, then something remains in theresiduals that can be removed by a more appropriate model. The residual error from thelinear model shows significant lack of fit (bad), while the quadratic, special cubic andfull cubic do not show significant lack of fit (good). At this point the quadratic modellooks very good.Lack of fit tableNow, scroll down to the last table: “Model Summary Statistics.” Here you see severalcomparative measures for model selection.Summary statisticsIgnoring the aliased cubic model, the quadratic model comes out best: low standarddeviation (“Std Dev”), high “R-Squared” statistics and low “PRESS.”Before moving on, you may want to print the Fit Summary tables by doing a File, Print.These tables, or any selected subset, can be also cut and pasted into a word processor,spreadsheet or any other Window’s application. You’re now ready to take an in-depthlook at the quadratic model.10 • Mixture Tutorial – Part 1 Design-Expert 7 User’s Guide