R help - Apr 2005 - AR1 in gls function

Dear R-project users

I would like to calculate a linear trend versus time taking into account a
first order autoregressive process of a single time series (e.g. data$S80
in the following example) using th gls function.

gls(S80 ~ tt,data=data,corAR1(value, form, fixed))

My question is what number to set in the position of value within corAR1?
Should it be the acf at lag 1?

I look forward for your reply

With kind regards

Prodromos Zanis




-- 
****************************************************
Dr. Prodromos Zanis
Centre for Atmospheric Physics and Climatology
Academy of Athens
3rd September 131, Athens 11251, Greece
Tel. +30 210 8832048
Fax: +30 210 8832048
e-mail: pzanis at geol.uoa.gr
Web address: http://users.auth.gr/~zanis/

On Fri, 15 Apr 2005, Prodromos Zanis wrote:
> Dear R-project users
>
> I would like to calculate a linear trend versus time taking into account a
> first order autoregressive process of a single time series (e.g. data$S80
> in the following example) using th gls function.
>
> gls(S80 ~ tt,data=data,corAR1(value, form, fixed))
>
> My question is what number to set in the position of value within corAR1?
> Should it be the acf at lag 1?
The initial value for the AR(1) parameter.  acf can mean autocovariance or 
autocorrelation: the value of the latter is a good starting point.
However, unless you have very high correlation (or are fixing the 
parameter), value=0 will usually work.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595