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. Thanks Simone Vincenzi -- Universita' degli Studi di Parma (http://www.unipr.it)
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
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-seriesof> 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 isthe> 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)
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-seriesof> 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 isthe> 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 at nemo.unipr.it -- -- Universita' degli Studi di Parma (http://www.unipr.it)