[R] ARMA and ARIMA modeling
edd at debian.org
Sat Mar 9 14:08:30 CET 2002
On Sat, Mar 09, 2002 at 07:18:17AM +0000, Prof Brian D Ripley wrote:
> Only ts has full handling of ARIMA models. It's changed a lot in R-devel.
> It also provides basic function on which all the others rely.
> tseries contains other models primarily of interest in economics, and
> GARCH models (which are better models of stock prices). It also has
Hm, "better" sounds odd here.
They cannot be compared directly: Garch models are different from Arma
models in the sense of being "arma-type models in the variance". They model
second moments (variance, and thus volatility), not first moments. Both have
benn combined by using AR(F)(I)MA for the first moment and Garch-type models
for the second moment. Doing that estimation jointly is tricky. It is also
tricky to fit the Ar(f)(i)ma model "well" to financial markets return.
Low-hanging fruits like this get picked up in the markets the earliest...
"Better" is of course correct in a less-narrow sense of Garch models being
considered a generally more valid fit in the variance space than arma models
in the mean space.
> arma, which is subsumed by arima (and arima0) in R-devel.
> fracdiff handles fractionally differenced models, a very specialized topic.
A generalisation of ARIMA by relaxing the d=1 restriction. I never had much
luck with the package -- the covariance matrix estimator seems very broken
(some elements are in the order of 1e-28, others 1e+20 which cannot be
sensible; try "example(fracdiff)".) I always meant to debug that but
Another somewhat important distinction is that David (or I) are not supposed
to use fracdiff or tseries at work -- fracdiff and some parts of tseries are
not free for commerical use.
Good judgement comes from experience; experience comes from bad judgement.
-- Fred Brooks
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