Dear All
I am attempting some power analyses, based on simulated data.
My experimental set up is thus:
Bleach: main effect, three levels (control, med, high), Fixed.
Temp: main effect, two levels (cold, hot), Fixed.
Main effect interactions, six levels (fixed)
For each main-effect combination I have three replicates.
Within each replicate I can take varying numbers of measurements
(response variable = Growth (of marine worms)) but, for this example,
assume eight). (I’m interested in changing this to see if the
experimental power changes much).
Total size = 3 x 2 x 3 x 8 = 144
The script thus far goes:
=========== start of script =================
library(lme4)
#Data frame structure
Bleach=rep(c("Control","Med","High"),each=48)
Temp= rep(rep(c("Cold","Hot"),each=24),3)
Rep= (rep(rep(rep(c("1","2","3"),each=8),2),3))
Ind= (rep(rep(rep(c(1:8),3),2),3))#not required for stats
#Fake data (based on pilot studies), only showing a single main effect
(bleach)
Growth=c( rnorm(48,3.27,0.77),rnorm(48,3.21,0.77),rnorm(48,3.64,1.17))
fake2=data.frame(Bleach,Temp,Rep,Ind,Growth);head(fake2)
#generate factor level for lmer as per Crawley, page 649
fake2$rep=fake2$Bleach:fake2$Temp:fake2$Rep#rep is used in the lmer
model
with(fake2,table(rep))#check that each rep contains 8 measurements
# run alternative (?equivalent) models
model1=aov(Growth~Bleach*Temp+Error(Bleach*Temp/Rep),data=fake2);summary(model1)
model2=lmer(Growth~Bleach*Temp+(1|rep),data=fake2);summary(model2)#note:
see above, rep<>Rep!
============ end of script ==========
I'd like to get familiar with using lme4 because it is likely that the
final results of the experiment will be unbalanced (which precludes the
use of aov I think). The df given by model1 seem to make sense. Any
guidance on any of the following would be much appreciated:
1. Are model1 and model2 equivalent?
2. For model1 - is the random component correctly specified and is
there a (simple) mechanism to get the appropriate F ratios and P
values?
3. For model2 - again, is the random component correct (probably not)
and why is the random effect (rep) variance and standard deviations so
low (zero in most iterations)?
4. For both models - how do I isolate (so I can tabulate and create
histograms) the appropriate P and/or t values? (for model2 - the
‘mer’ object doesn’t seem to contain the t values but maybe
I’m missing something).
Direction to any more generic sources of information regarding power
analysis in hierarchical models would be gladly received.
Thank you
Tom.
-------------------------------------------------------------------------
Tom Wilding, MSc, PhD, Dip. Stat.
Scottish Association for Marine Science,
Scottish Marine Institute,
OBAN
Argyll. PA37 1QA
United Kingdom.
Phone (+44) (0) 1631 559214
Fax (+44) (0) 1631 559001
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