[R] Nested Designs

[Alan T. Arnholt]

Dear R list,

What is the proper way to specify a nested model so that
the F values agree with the expected mean square errors?
Specifically, suppose I have a design where "Heads" are 
nested within "Machines".  I would like to model the 
following Y_ijk = Mu + Machine_i +Head_j(i) +Error_k(ij).  

Using the commands below, 

> summary(aov(Strain~Machine + Head%in%Machine ))
             Df Sum Sq Mean Sq F value  Pr(>F)  
Machine       4  45.07   11.27  1.0532 0.38762  
Machine:Head 15 282.87   18.86  1.7625 0.06252 .
Residuals    60 642.00   10.70    

the F value to test "Machine" uses the MS residual 
instead of MS Head(Machine).

When I use the following no F value is reported

> summary(aov(Strain~Machine + Head%in%Machine + Error(Machine)))

Error: Machine
        Df Sum Sq Mean Sq
Machine  4 45.075  11.269

Error: Within
             Df Sum Sq Mean Sq F value  Pr(>F)  
Machine:Head 15 282.88   18.86  1.7625 0.06252 .
Residuals    60 642.00   10.70  

Is the second approach the best I can do?

Thanks  ALAN 

___________________________________
Alan T. Arnholt
Office phone: (828) 262 - 2863
Office fax  : (828) 265 - 8617

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