[R] fractional factorial design in R

From: Ulrike Grömping <groemping_at_tfh-berlin.de>
Date: Tue 24 Jan 2006 - 02:19:03 EST


>Roberto Furlan wrote:
>
>Hi,
>i need to create a fractional factorial design sufficient to estimate the
>main effects.
>The factors may have any number of levels, let's say any number from 2 to 6.
>I've tried to use the library conf.design , but i cannot figure out how to
<write the code.
>For example, what is the code for a design with 5 factors (2x3x3x5x2) and
>only main effects not confounded?
>
>thanks in advance!
>Roberto Furlan
>University of Turin, Italy

Hi Roberto,

unfortunately, fractional factorial designs typically require all factors to have the same number of levels. Hence, your 2x3x3x5x2 example is not a simple special case of a fractional factorial design.

There are some special plans for mixed level designs, but the conf.design function requires all factors to have the same number of levels, as you can also find in its help:
    p: The common number of levels for each factor.  Must be a prime           number.

ffDesMatrix from package BHH2 is even worse, since it requires all factors to have 2 levels:

    k: numeric. The number of 2-levels design factors in the

          designs.

(Note that designs with two level-factors only can accommodate a few fourlevel  factors, by combining two main effects plus their interaction for the three degrees of freedom of the 4-level factor. For doing this, I would recommend to have someone plan the design who really knows what he/she is doing.)

Regards, Ulrike



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