[R-sig-ME] unreplicated repeated measures

Christine Griffiths Christine.Griffiths at bristol.ac.uk
Tue Jun 23 19:29:42 CEST 2009

Dear R users,

Sorry if this question is not applicable to this site. I am having problems 
analysing unreplicated repeated measures. I calculated food web properties 
for three treatments over time (10 months). It is unreplicated in that I 
only have one observation per month per treatment. My problem is that I am 
interested in how a food web property varies between Treatments and over 

Originally I had tried using lmer:
but this provides the following error for which I have not found an 
explanation to on the R site.
Error in mer_finalize(ans) : Calculated PWRSS for a LMM is negative

I suspect it is because I had treated month as a factor and consequently 
due to the lack of replication and trying to interact these two categorical 
variables it fails. I can overcome this problem by treating month as a 
continuous variable from which I calculated confidence intervals using MCMC 
method. However I am not sure how these are being calculated and if this is 
accurate, given I have no mean per Treatment as such. Is it acceptable to 
use month as a continuous variable?

Alternatively, I tried using repeated measures ANOVA, aov, to model the 
data. I am cautious to use this method as it indicates significant 
differences which are not apparent from the plotted raw data. Given that my 
data lacks replication, I am wary of this method.

I have investigated time series analysis, but I am reluctant to venture 
down this route.

Any reassurance or advice as to the best technique given my data would be 
greatly appreciated.

Many thanks,


More information about the R-sig-mixed-models mailing list