I'm trying to use tapply to output means and SD or SE for my data but 
seem to be limited by how many times I can subset it.  Here's a snippet 
of my data

 > stems353[1:10,]
      Time DataSource   Plot Elevation Aspect Slope     Type Species 
SizeClass Stems
1  Modern    Cameron 70F221      1730    ESE    20  Conifer    ABCO    
Class1     3
2  Modern    Cameron 70F221      1730    ESE    20  Conifer    ABMA    
Class1     0
3  Modern    Cameron 70F221      1730    ESE    20 Hardwood    ACMA    
Class1     0
4  Modern    Cameron 70F221      1730    ESE    20 Hardwood    AECA    
Class1     0
5  Modern    Cameron 70F221      1730    ESE    20 Hardwood    ARME    
Class1     0
6  Modern    Cameron 70F221      1730    ESE    20  Conifer    CADE    
Class1    15
7  Modern    Cameron 70F221      1730    ESE    20 Hardwood    CELE    
Class1     0
8  Modern    Cameron 70F221      1730    ESE    20 Hardwood    CONU    
Class1     0
9  Modern    Cameron 70F221      1730    ESE    20  Conifer    JUCA    
Class1     0
10 Modern    Cameron 70F221      1730    ESE    20  Conifer    JUOC    
Class1     0

I'd like to see means/SD of "Stems" stratified by "Species", "Time" and 
"SizeClass".  I can get R to give me this for means by species:

 > tapply(stems353$Stems, stems353$Species, mean)
         ABCO         ABMA         ACMA         AECA         
ARME         CADE         CELE
0.7305240793 0.8569405099 0.0003541076 0.0010623229 0.0017705382 
0.4684844193 0.0063739377
         CONU         JUCA         JUOC         LIDE         
PIAL         PICO         PIJE
0.0017705382 0.0003541076 0.0959631728 0.0138101983 0.3905807365 
1.5651558074 0.2315864023
         PILA         PIMO        PIMO2         PIPO         
PISA         POTR         PSME
0.1774079320 0.1880311615 0.0311614731 0.6735127479 0.0237252125 
0.0506373938 0.2000708215
         QUCH         QUDO         QUDU         QUKE         
QULO         QUWI        Salix
0.0474504249 0.1203966006 0.0000000000 0.2071529745 0.0003541076 
0.0548866856 0.0003541076
         SEGI         TSME
0.0021246459 0.5017705382
 >

but I really need to see each species by SizeClass and Time so that each 
value would be labeled something like "ABCOSizeClass1TimeModern".  
Adding 2 variables to the function doesn't seem to work

 > tapply(stems353$Stems, stems353$Species, stems353$SizeClass, 
stems353$Time, mean)
Error in match.fun(FUN) :
   'stems353$SizeClass' is not a function, character or symbol

I've already created proper subsets for each of these groups, e.g. one 
subset is called "stems353ABCO1" and I can run analyses on this.  But, 
trying to extract means straight from those subsets doesn't seem to work

 > mean(stems353ABCO1)
[1] NA
Warning message:
In mean.default(stems353ABCO1) :
   argument is not numeric or logical: returning NA
 >

Thanks,
Chris Dolanc

-- 
Christopher R. Dolanc
PhD Candidate
Ecology Graduate Group
University of California, Davis
Lab Phone: (530) 752-2644 (Barbour lab)


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