GLM Syntax - Example 8: Mixture/Process Design
This example illustrates the specification for a mixture surface design that includes a process (non-mixture component) variable. You can run this example with the example data file Fish2.sta. Two alternative Design specifications are shown; one using the | (bar) and "-" (deletion) operator shortcuts, the other with the terms of the response surface model explicitly written out. This example illustrates the usefulness (efficiency) of the deletion operator for specifying mixture-process designs. A special cubic polynomial model is fitted to the mixture components; for a discussion of these data and this analysis, see Cornell (1990b). For a general discussion of mixture designs, see the Experimental Design module. Note that brief descriptive comments are enclosed in curly brackets.
GLM;
{ Dependent variable list: }
DEPENDENT = RATING;
{ Specification of the process (grouping) variable; note that
no codes (values) are specified, so the program will by default
take all grouping values found in the data file. }
GROUPS = TEMPERATURE;
{ The mixture components are specified as continuous predictors. }
COVARIATE = MULLET SHEEPSHD CROAKER;
{ Here the bar operator is used to construct the full factorial
design; the bar operator will evaluate to all main effects and
interactions (up to 4-way) between the grouping variables.
Then the deletion operator (-) is used to remove the TEMPERATURE
main effect from the model; see Cornell (1990b) for details. }
DESIGN = TEMPERATURE |
MULLET | SHEEPSHD | CROAKER
- TEMPERATURE;
{ An alternative Design specification without the use of the | and - operators is:
DESIGN = MULLET + SHEEPSHD + CROAKER +
TEMPERATURE*MULLET + TEMPERATURE*SHEEPSHD +
MULLET*SHEEPSHD + TEMPERATURE*CROAKER +
MULLET*CROAKER + SHEEPSHD*CROAKER +
TEMPERATURE*MULLET*SHEEPSHD +
TEMPERATURE*MULLET*CROAKER +
TEMPERATURE*SHEEPSHD*CROAKER +
MULLET*SHEEPSHD*CROAKER +
TEMPERATURE*MULLET*SHEEPSHD*CROAKER; }
{ The intercept is excluded from the model because of the mixture
constraint; see Experimental Design for details. }
INTERCEPT = EXCLUDE;
For more examples, see GLM Syntax Examples.