= Factorial contrast
{title2=$\chi_A(x)=\prod_{j\in A}x_j$}
In a two-level <factorial design>, a factor subset defines a sign column. The coefficient of this column is its response <contrast> divided by the number of runs when the columns are <orthogonal>. A singleton describes a <main effect> and a pair describes a <two-factor interaction>. On the full cube, distinct columns have zero <inner product>: summing over any coordinate in their <symmetric difference> cancels the terms.
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