Đây là phạm vi của các biến thể trong si cho n = 3 kích thước mẫu. Một so sánh chính thức của sự bình đẳng của hai phương sai mẫu sử dụng các số liệu thống kê F. So sánh hai phương sai mẫu, mỗi ước tính với ba bậc tự do, sẽ sử dụng giá trị 5% trên F3, 3 = 9,28 | TABLE Experimental Design and Results for Iteration 2 C g L D 1 h R g h c ễ o Q Iteration 2 Optimum D _ n n to 1----------------r Phenol Concentration mg L FIGURE Approximation of the response surface estimated from the second-stage exploratory experiment. CD term was zero. Figure the response surface indicates that C should be reduced and that further increase in D may be beneficial. Before moving the experimental settings consider the available information more carefully. The fitted model describes a plane and the plane is almost horizontal as indicated by the small coefficients of both C and D. One way we can observe a nearly zero effect for both variables is if the four corners of the 22 experimental design straddle the peak of the response surface. Also the direction of steepest ascent has changed from Figure to . This suggests that we may be near the optimum. To check on this we need an experimental design that can detect and describe the increased curvature at the optimum. Fortunately the design can be easily augmented to detect and model curvature. Third Iteration Exploring for Optimum Conditions Design We anticipate needing to fit a model that contains some quadratic terms such as R b0 b1C b2D b12CD bnC2 b22D2. The basic experimental design is still a two-level factorial but it will be augmented by adding star points to make a composite design Box 1999 . The easiest way to picture this design is to imagine a circle or ellipse depending on the scaling of our sketch that passes through the four corners of the two-level design. Rather than move the experimental region we can use the four points from iteration 2 and four more will be added in a way that maintains the symmetry of the original design. The augmented design has eight points each equidistant from the center of the design. Adding one more point
