[R] fractional factorial design in R
Ulrike Grömping
groemping at tfh-berlin.de
Mon Jan 23 16:19:03 CET 2006
>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 four-
level 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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