Augment Factorial to CCD

Runs per axial (star) point: this is the number of times each axial run will be performed. One is the default. If you have concerns that you will not get an accurate value at this point, you may consider increasing this to two.

Center points: the recommended number of center points is given. Please don’t reduce this number. Because a central composite is usually centered near the likely optimum, more runs at the center give better predictions in this area. Also, the replicates provide a lack of fit test. Remember that center point runs should be randomly performed throughout the design.

Alpha values: these determine the position of the axial runs

  • Rotatable (k<6): the default setting for up to 5 factors, this creates a design that has the standard error of predictions equal at points equidistant from the center of the design.

  • Orthogonal Blocks: this alpha value allows each term to be estimated independently of the blocks.

  • Spherical: this puts all factorial and axial points on the surface of a sphere of radius = square root of k (the number of factors)

  • Orthogonal Quadratic: this provides alpha values that allow the quadratic terms to be independently estimated from the other terms.

  • Practical (k>5): this is the default for designs that have 6 or more factors. The alpha value is the 4th root of the number of factors. This has been shown to produce axial values that can practically be run, and yet the design still has sound statistical properties.

  • Face Centered: pull the axial points into the faces of the cube - at +/- 1 levels. This produces a design where each factor only has 3 levels.

  • Other: specify any alpha value desired. We recommend checking Design Evaluation before completing the runs.

  • Put runs into block: there is a choice of new block or one of the current blocks.