Random source of error in the split-split plot design

[Ahmed_Attia]

Hi All,

I have a question about error source in split-split plot design. I have combined two years of data as follows;

class Year Rep Irrig Tillage Variety;

model HJul24 =  Irrig|Tillage|Variety;

Random  Rep(Year) Rep(Year Irrig)  Rep(Year Irrig Tillage);

Irrig was the main plot, Tillage was the subplot, and Variety was the sub-subplot in three replicates. I would like to consider year in the random model. My question is; should I include the Tillage in the random model or not. As I will say in the paper;

Irrigation, tillage, variety, and their interaction were considered as fixed effects. Year, replicate, and their interaction with irrigation and tillage were considered as random effects.

Thank you very much.

Ahmed Attia

[SteveDenham]

Hi Ahmed,

If you have sufficient data to estimate the variance components with tillage included, I would put them into the random statement.  However, I would be very surprised if the message "G matrix is not postive definite" or some such does not appear.  The reason for still including them is that they are part of the design, and the degrees of freedom for tests should reflect the "skeleton ANOVA" (see Stroup's Generalized Linear Mixed Models for more on this).

Steve Denham

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