[R] Help with short time series

From: Simone Vincenzi <simone.vincenzi_at_nemo.unipr.it>
Date: Wed 09 Aug 2006 - 07:44:49 EST


Thanks for the help.
I provide below the dataset I'm using, it's a little bit different from what I was describing (sorry for that). The streams are 3 and I have an unequal number of years for each stream.

Stream Density Year

1     Zak    0.20 2000 
2     Zak    0.36 2001 
3     Zak    0.41 2002 
4     Zak    0.34 2003 
5     Zak    0.28 2004 
6     Gor    0.08 1999 
7     Gor    0.05 2000 
8     Gor    0.14 2001 
9     Gor    0.16 2002 
10    Gor    0.13 2003 
11    Gat    0.18 2004 
12    Gat    0.10 2001 
13    Gat    0.37 2002 
14    Gat    0.57 2003 
15    Gat    0.47 2004

I tried to follow the suggestions of Dieter, but the model does not fit. Any suggestion will be appreciated

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

-- 
Universita' degli Studi di Parma (http://www.unipr.it)

_________________________________________
Simone Vincenzi, PhD Student 
Department of Environmental Sciences
University of Parma
Parco Area delle Scienze, 33/A, 43100 Parma, Italy
Phone: +39 0521 905696
Fax: +39 0521 906611
e.mail: svincenz@nemo.unipr.it 


-- 



 


--
Universita' degli Studi di Parma (http://www.unipr.it)

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Received on Wed Aug 09 07:59:49 2006

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