[Rd] CCF and ACF

[Simone Giannerini]

Thank you Gabor, by the way I have checked in the Matlab GARCH
toolbox, the crosscorrelation function is reversed with respect to R,
I think it would be useful for the users to have documented the way
CCF is computed.

Simone

On 10/2/06, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> Using "ts" objects these can be written in the following consistent
> manner:
>
> > sum(ts(x) * lag(ts(x)) / sum(ts(x)^2))
> [1] -0.2648633
>
> > sum(ts(x) * lag(ts(y)) / sqrt(sum(ts(y)^2) * sum(ts(x)^2)))
> [1] 0.5930216
>
> On 10/2/06, Simone Giannerini <sgiannerini at gmail.com> wrote:
> > Dear all,
> >
> >  given two numeric vectors x and y, the ACF(x) at lag k is
> > cor(x(t),x(t+k)) while the CCF(x,y) at lag k is cor(x(t),y(t-k)). See
> > below for a simple example.
> >
> >  > set.seed(1)
> >  > x <- rnorm(10)
> >  > y <- rnorm(10)
> > > x
> >  [1] -0.6264538  0.1836433 -0.8356286  1.5952808  0.3295078 -0.8204684
> >  0.4874291  0.7383247  0.5757814 -0.3053884
> > > y
> >  [1]  1.51178117  0.38984324 -0.62124058 -2.21469989  1.12493092
> > -0.04493361 -0.01619026  0.94383621  0.82122120  0.59390132
> > > x <- x - mean(x);
> > > y <- y -mean(y);
> >
> >   ## ACF OF x(t)
> > > print(acf(x,lag=2,plot=F),digits=5))
> >
> >  Autocorrelations of series 'x', by lag
> >
> >        0        1        2
> >  1.00000 -0.26486 -0.25352
> >
> >  ## For Instance ACF(x) at lag 1 is Corr(x(t),x(t+1)):
> >
> >  > sum(x[1:9]*x[2:10])/sum(x^2)
> > [1] -0.2648633
> >
> >  ## whereas CCF(x,y) at lag 1 is Corr(x(t),y(t-1))
> >  > ccf(x,y,lag=2,plot=F)
> >
> > Autocorrelations of series 'X', by lag
> >
> >    -2     -1      0      1      2
> >   -0.139  0.593 -0.377 -0.382  0.116
> >
> >  > sum(x[1:9]*y[2:10])/sqrt(sum(x^2)*sum(y^2))
> >
> > [1] 0.5930216
> >
> > > sum(x[2:10]*y[1:9])/sqrt(sum(x^2)*sum(y^2))
> >  [1] -0.3822885
> >
> > from a quick survey on textbooks (Brockwell and Davis, Chatfield,
> > Fuller) it looks like different authors use different conventions so
> > that I think that it would be nice to clarify this in the
> > documentation.
> >
> > Ciao
> >
> > Simone
> >
> > > R.version
> >               _
> > platform       i386-pc-mingw32
> > arch           i386
> > os             mingw32
> > system         i386, mingw32
> > status         beta
> > major          2
> > minor          4.0
> > year           2006
> > month          09
> > day            20
> > svn rev        39421
> > language       R
> > version.string R version 2.4.0 beta (2006-09-20 r39421)
> >
> > > Sys.getlocale()
> > [1] "LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
> > States.1252;LC_MONETARY=English_United
> > States.1252;LC_NUMERIC=C;LC_TIME=English_United  States.1252"
> >
> >
> > ______________________________________________________
> >
> > Simone Giannerini
> >  Dipartimento di Scienze Statistiche "Paolo Fortunati"
> > Universita' di Bologna
> > Via delle belle arti 41 - 40126  Bologna,  ITALY
> >  Tel: +39 051 2098262  Fax: +39 051 232153
> >
> > ______________________________________________
> > R-devel at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>


-- 
______________________________________________________

Simone Giannerini
Dipartimento di Scienze Statistiche "Paolo Fortunati"
Universita' di Bologna
Via delle belle arti 41 - 40126  Bologna,  ITALY
Tel: +39 051 2098262  Fax: +39 051 232153