Specifications for Mixture Variables
Select the User-defined values (high/low factor values) option button on the
Design & Analysis of Mixture Experiments - Analyze Design tab (or click the adjacent 
button), or click the Change factor names, values, etc. button on the
Design of a Mixture Experiment - Quick tab or
Display design tab to display the Specifications for Mixture Variables spreadsheet with (1) the current factor (component) names, (2) minimum and maximum values, and (3) the sum total for the mixture. Enter the desired text and numeric values, and then click the OK button; the values that you entered will then be used in the spreadsheet displaying the design (option Summary: Display design on the
Design of a Mixture Experiment - Quick tab).
- Factor Name
- Enter factor names in the Factor Name column. If you save the spreadsheet as a data file, then the respective factor names will become variable names in the Statistica data file.
- Low Value
- Enter the value for the low setting of the respective factor in the Low Value column.
- High Value
- Enter the corresponding value for the high setting of the respective factor in the High Value column.
- OK
- Click the OK button to accept the values entered in the Specifications for Mixture Variables spreadsheet and return to the previous dialog.
- Cancel
- Click the Cancel button to return to the previous dialog. Any entries made in the Specifications for Mixture Variables spreadsheet will be ignored.
- Lower constraints
- The standard simplex-lattice and simplex-centroid designs require vertex points, that is, pure blends consisting of only one ingredient. In practice, those points may often not be valid, that is, pure blends cannot be produced because of cost or other constraints. For example, suppose in a 3-component example, component B is constrained so that xB >= 200. The total of the 3-component mixture must be equal to 800. This constraint can be visualized in a triangular graph by a line at the triangular coordinate for B=200, that is, a line that is parallel to the side opposite to the B vertex point. Now, to construct the standard design in the sub-triangle, the maximum values for components A and C must be adjusted. (Later, in the analysis, you can review the parameter estimates for the so-called pseudo-components, i.e., treating the constrained triangle as if it were a full triangle.)
After you entered the lower and upper values for the three components (0 <= xA <= 800; 200 <= xB <= 800; 0 <= xC <= 800, Total = 800) into the Specifications for Mixture Variables spreadsheet, and then click the OK button, the following message will be displayed:
The selected specifications (highs, lows, and mixture total) are inconsistent and have to be adjusted to produce a standard design; do you want to adjust the factor highs?
If you click Yes, then the Specifications for Mixture Variables spreadsheet will be displayed again, with the adjusted values that permit Statistica to construct the standard design in a sub-triangle (or sub-simplex). Specifically, when there are lower constraints for one or more of the components, Statistica will adjust the High Value for the other components, so that:
x'high,i = Total-Li
Here, x'high,i stands for the adjusted High Value for the i'th component, Total stands for the mixture total, and Li stands for the sum of the lower constraint (limit) for all components excluding component i.
Upper- and lower-bound restrictions. When there are both upper and lower constraints (as is often the case in experiments involving mixtures), then the standard simplex-lattice and simplex-centroid designs can no longer be constructed, because the subregion defined by the constraints is no longer a triangle. In that case, use the option Designs for constrained surfaces and mixtures to find the vertex and centroid points for the constrained region in the full simplex.
- OK
- Click the OK button to accept the values entered in the Specifications for Mixture Variables spreadsheet and return to the previous dialog.
- Cancel
- Click the Cancel button to return to the previous dialog. Any entries made in the Specifications for Mixture Variables spreadsheet will be ignored.