Standard least squares fits mixture models using the parameterization suggested in Scheffé (1958). The parameters for this model cannot easily be used to judge the effects of the mixture components. The Cox Mixture model is a reparameterized and constrained version of the Scheffé model. Using its parameter estimates, you can derive factor effects and the response surface shape relative to a reference point in the design space. See Cornell (1990) for a complete discussion.
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Select Analyze > Fit Model.
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Select Macros > Mixture Response Surface.
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Click Run.
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From the red triangle menu next to Response Y1, select Estimates > Cox Mixtures.
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Figure 2.39 Cox Reference Mixture Window
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Click OK.
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Figure 2.40 Cox Mixtures