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In central composite designs, there are three kinds of points involved:
Factorial points: A two level full or fractional factorial design, using up to nine factors, is embedded in the central composite design. The points associated with this embedded design are also called "corner points," and are indicated by a 1 in the Point Type column on the Data tab.
Axial points: For each factor, two axial points are calculated as follows: all other factors are set to their coded 0 value (i.e., midway between the low and high — or coded -1 and 1 — values) and the factor in question is set to -alpha and +alpha, where alpha is a value specified by the user. Axial points, sometimes called "star" points, allow estimation of curvature. These points are indicated by a -1 in the Point Type column on the Data tab.
Center point: All factors are set to the coded 0 value. These points are indicated by a 0 in the Point Type column on the Data tab.
Thus, for each factor, five values are used: -alpha, -1, 0, 1, +alpha.
The following topics describe how to work with central composite designs, including:
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