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**HDIR(***matrix1, matrix2***)**

where *matrix1* and *matrix2*
are numeric matrices or literals.

The HDIR function performs a direct product on all
rows of *matrix1* and *matrix2* and creates a
new matrix by stacking these row vectors into a matrix.
This operation is useful in constructing
design matrices of interaction effects.
The *matrix1* and *matrix2*
arguments must have the same number of rows.
The result has the same number of rows
as *matrix1* and *matrix2*.
The number of columns is equal to the product of the
number of columns in *matrix1* and *matrix2*.

For example, the statements

a={1 2, 2 4, 3 6}; b={0 2, 1 1, 0 -1}; c=hdir(a,b);produce a matrix containing the values

C 3 rows 4 cols (numeric) 0 2 0 4 2 2 4 4 0 -3 0 -6The HDIR function is useful for constructing crossed and nested effects from main effect design matrices in ANOVA models.

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