[R] How to specify the hierarchical structure of a split plot using lmer ??

[Søren Højsgaard]

I have a problem getting the lmer function of the lme4 package to use the
appropriate degrees of freedom for testing. Consider the Semiconductor data
from the SASmixed package:
 
library(SASmixed)
Semi.lme <- lme(resistance ~ ET * position, random=~1|Grp, data=Semiconductor)
anova(Semi.lme)
             numDF denDF  F-value p-value
(Intercept)       1    24 3237.261  <.0001
ET                  3     8    1.942  0.2015
position          3    24    3.385  0.0345
ET:position     9    24    0.809  0.6125
 
Here, the ET effect is (correctly) tested on 8 denominator degrees of freedom.
In the example in the SASmixed package, the following code is presented:
 
(fm1Semi <- lmer(resistance ~ ET * position + (1|Grp), Semiconductor))
anova(fm1Semi)
Analysis of Variance Table
                Df Sum Sq Mean Sq  Denom F value  Pr(>F)
ET              3  0.647   0.216 32.000  1.9415 0.14273
position       3  1.129   0.376 32.000  3.3855 0.02991 *
ET:position  9  0.809   0.090 32.000  0.8092 0.61127
 
So here, all effects are tested with 32 denominator degrees of freedom.
I have looked at the help page for lmer but have been unable to figure out
how to specify the hierarchical structure of a split plot experiment. Also,
I have Googled but without any luck. Any help will be appreciated.
Søren