[R]: use of Error() for repeated measures with more than 2 factors
Peter Ho
peter at esb.ucp.pt
Wed Aug 14 17:15:03 CEST 2002
I have been trying to analyse an unreplicated repeated measures 2-level
factorial design with 11 factors using aov() with Error(), similar to
that described in "Notes on the use of R for psychology experiments and
questionnaires" by Jonathan Baron and Yuelin Li (see the example of Hay's)
I have found that there seems to be a limit, in the number of factors in
Error() . For example, the following worked :
aov(CV ~(Twb+Plb+Psb+tinsb+tseedb+Tmb+Mcb+Mom0b+Mom1b+Mom2b+Mom3b)^3 +
Error(Batch /
(Twb+Plb+Psb+tinsb+tseedb+Tmb+Mcb)),simulation.6batches.aovdata)
but not:
aov(CV ~(Twb+Plb+Psb+tinsb+tseedb+Tmb+Mcb+Mom0b+Mom1b+Mom2b+Mom3b)^3 +
Error(Batch /
(Twb+Plb+Psb+tinsb+tseedb+Tmb+Mcb+Mom0b+Mom1b)),simulation.6batches.aovdata)
which I suppose should be the right way to code the strata.formula for
11 fixed factors. Returned the following reply after running this script:
>Error in eval(expr, envir, enclos) : couldn't find function "Error"
The only difference between the two scripts is the second one contains
all 11 factors. Is this a bug? Or should I be using lme() or some other
function when the number of factors is greater than the "limit"?
I would also like to hear some views as to whether Error() should be
used for repeated measures, pro & cons,etc? I have found differing views
in books. For example, Neter et al "applied linear statistical models "
uses simply the combined treatment by subject interaction mean square
when testing main effects and their interactions. They do state that the
relaxation of the assumptions of a repeated measures model can be made
by the inclusion of first-order interaction between each factor and
subjects in the model. However, this still falls short of the ideas
suggested with using Error() for testing.On the other hand, Mead's "the
design of experiments puts repeated measures in the same chapter with
split unit design, which are multistratum design. Jonathan Baron's and
Yuelin Li's document also references a number of books on this matter.
I know that this point is not really suitable for r-help, but it is
related indirectly with regard to the use of strata.formula in Error().
Any comments would be grateful
Peter Ho
ISR-Porto
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