getAmatrix finds A-matrices used to express ANOVA sum of squares as quadratic forms in y.
(matrix) lower order model matrix (comprising all factors entering model before 'X2'-factor)
(matrix) higher order model matrix (entering model after 'X1' factors)
Compute A-Matrix for expressing ANOVA Type-1 sequential sum of squares SS as quadratic form in y (column vector of observations), y' * A * y = SS. In random models, when the intercept is the only fixed effect, the design matrix of fixed effects is equal to a column vector where each element is equal to one (1). In this setting, it holds 1' * A * 1 = 0, i.e. all elements of A summing to zero.
This function is not intended to be used directly.
(matrix) for obtaining quadratic forms in y expression ANOVA sum of squares
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