On 01/31/2012 11:34 PM, swertie wrote:
Hello!
I have a dataset with monthly temperatures for 4 different years and I would
like to test if they are significantly different between the years. As I
collected the data for different sites I wondered if there were some
possibility to perform the calculation at once or if I have to repeat it for
each site.
I tried like this:
Model_Temp<- lmer(Temp ~Year + (1|Site)+(1|Month),na.action=na.omit, data=
MeanTemp)
But I am not sure at all if it is correct to put the sites as random factors
or if I have to do the calculation for each site separately. I am not
interested in correlation between sites, but only between years for each
different site.
Alternatively, can you indicate me some way to plot this?
Hi swertie,
Let's say you have monthly temperature readings for four sites:
mon_temp<-data.frame(site=rep(LETTERS[1:4],48),
year=rep(2008:2011,each=48),month=rep(month.abb,each=4),
temp=20+rnorm(192)+
10*sin(rep(rep(seq(1.5*pi,3.5*pi,length.out=12),each=4,4))))
Having gotten some data, you can then display a line plot of the
temperatures for the sites:
plot(mon_temp$temp[mon_temp$site=="A"],type="b",xaxt="n",ylim=c(0,40),
main="Temperature by site",xlab="Year",ylab="Temperature")
axis(1,at=seq(6,42,by=12),labels=2008:2011)
lines(mon_temp$temp[mon_temp$site=="B"],type="b",pch=2,col=2)
lines(mon_temp$temp[mon_temp$site=="C"],type="b",pch=3,col=3)
lines(mon_temp$temp[mon_temp$site=="D"],type="b",pch=4,col=4)
legend(24,40,LETTERS[1:4],pch=1:4,col=1:4)
Unless you have some reason to think that the sites vary in some way
that would affect temperature, for instance altitude, using site as a
factor in an overall model might not get you much. Also you may want to
test yearly averages, as there is a strong seasonal component. Thus you
might want to think about a time series model.
Jim
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