Table 13.11 has several interesting properties:
1. Except for the identity column I, each column has an equal number of plus and minus
signs.
2. The sum of products of signs in any two columns is zero; that is, the columns in the
table are orthogonal.
3. Multiplying any column by column I leaves the column unchanged; that is, I is an identity
element.
4. The product of any two columns yields a column in the table; for example, A × B = AB,
and AB × ABC = A2B2C = C, since any column multiplied by itself is the identity column.
The estimate of any main effect or interaction is determined by multiplying the factorlevel
combinations in the first column of the table by the signs in the corresponding main
effect or interaction column, adding the result to produce a contrast, and then dividing the
contrast by one-half the total number of runs in the experiment. Expressed mathematically,
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