中心组合设计选择alpha的问题

细履平沙

Selecting the Value of Alpha for a CCD
The value of alpha determines the location of the star points（星点） in a central composite design. It’s expressed in terms of the coded values assigned to the low and high levels of the factors: from -1 to 1（它以编码的形式表示，最低值最高值分别赋值为-1和+1）. Alpha is usually somewhat larger than 1.

There are several ways of calculating an appropriate alpha, based on the statistical property you want to optimize. （几种确定alpha值的方法）

Practical:

Orthogonal Quadratic（正交二次模型）:Another alpha value applies when the design is blocked. It will try to make the blocks orthogonal to the independent factors in the second degree model so the blocks will not influence their determination. Orthogonality means that the model terms and block effects can be estimated independently with no correlation.

Rotatable（可旋转的）: Rotatability implies that the variation in the response predicted by a model will be constant at a given distance from the center of the design. You will see circular contours on the standard error plot of rotatable designs. (To view this, click on Design Evaluation, Graph.)

Spherical（球形的）: This will produce a design where the axial points lie in a sphere with the factorial points.（这样可以使轴点（星点）位于因素点形成的球体内部）

Face Centered:Setting alpha equal to 1 creates a Face-centered central composite design. This is desirable because it is only a three-level design, and ensures that the axial runs will not be any more extreme values than the factorial portion.（当alpha 等于1时，星点位于表面中心。这样的设计适用于三水平，这样保证轴向水平实验不会极端化）

monkey2007

学习了，不过不太明白，呵呵！

kxlynn

我也是，看了也不太懂
alpha它是什么意思，为什么有的时候是-2（四因素三水平）有点时候是-1.68呢？

lyw84102993

还是不明白！！