'Block' when assesing an existing design

[nashkato]

I have an existing design with 360 rows, 40 blocks (9 questions per respondent). This was blocked previously.

When I import to NGene for evaluation, if I use the 'block' option, NGene creates new blocks. How does it do this? By what criteria? They are quite different than the ones used in the existing design.

Thanks.

[johnr]

dear Nashkato

Blocking involves attempting to locate an additional column which is orthogonal to the remainder of your design attributes. If your design attributes are orthogonal, this may be possible. When you let go of orthogonality, the design attributes are no longer uncorrelated amongst each other and it will generally not be possible to locate an orthogonal blocking column that is orthogonal to a non-orthogonal set of columns.

To locate a blocking column for a non-orthogonal design, Ngene randomly searches through blocking columns and attempts to minimise the correlation between the blocking column and the design columns. Given that it will use different seeds, it will locate different blocking columns each time, unless you use a larger number of iterations in the sorting (which at some point converge to the same solution). You can change the default values in the blocking column search. This is described in Section 11.3.4 of the manual.

John