Re: [R] Help with short time series

From: Dieter Menne <dieter.menne_at_menne-biomed.de>
Date: Mon 07 Aug 2006 - 19:53:11 EST

Simone Vincenzi <simone.vincenzi <at> nemo.unipr.it> writes:

>
> Dear R-list,
> I have a statistical problem with the comparison of two short time-series of
> density data in an ecological framework. I have to compare two short time
> series (5 years, one value for each year) of species density data (it is the
> density of fish in two different streams) to test if the two means of the
> five densities are significantly different, so basically if the two mean
> stream-specific fish densities are significantly different.
> I don't think I can use a straight t-test due to the problem of
> autocorrelation and I don't think I can use a repeated measure ANOVA as I
> don't have any replicates.
> Any help would be greatly appreciated.

try something like

library(nlme)
summary(lme(dens~stream+year,data=mystreamdata,random=~year|stream))

This should also give you an estimate if the slopes are different if you test against the simplified model

summary(lme(dens~stream+year,data=mystreamdata,random=~1|stream))

Since you did not provide a short example data set, this is only approximatively right.

Dieter



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