# st: Seasonal Variables-Differencing or Dummies

 From "REINERTSON,M YVONNE" To statalist@hsphsun2.harvard.edu Subject st: Seasonal Variables-Differencing or Dummies Date Sun, 8 May 2005 12:02:31 -0400 (EDT)

```Dear Nick and Thuy,

I have decided to begin my study of seasonality with the purchase
of the
Ghysels & Osborn book. I thank you both for your help and for the
given suggestions.

Yvonne

I am probably missing the point of this
question, but I don't think you can list the circumstances
in which S. is useful or valid. For example,

use http://www.stata-press.com/data/r9/air2.dta, clear
tsline S.air

I just used S. to help visualize the non-trend
component, mostly seasonal.

In short, S. is likely to be useful
whenever you want it. No model need be in sight.

There is a vast book on time series by Hipel
and McLeod with environmental applications.
I'm pretty sure they discuss seasonality.
Franses is another author who springs to mind.
A search on Amazon would no doubt yield others.

Nick
n.j.cox@durham.ac.uk

These two books specifically deal with seasonality:
1/ Periodic time series model, Franses & Paap, 2004.
2/ The econometric analysis of seasonal time series, Ghysels &
Osborn, 2001.
Another book, which is not dedicated to seasonality but has good
discussion
about seasonality is Applied econometric time series, Enders,
2004.
Even though they are not using Stata for their examples, check
them out if
you need to know how to handle seasonality.
Best,
TL

To my original message:

Dear Statalisters,

to use seasonal variables in time-series regressions. My very
basic understanding of the correct use of the seasonal
differencing technique (time series operator _S._ in Stata) is in
the autoregressive integrated moving average (ARIMA) models. For
other regression techniques in analyzing time-series, and panel
data, the seasonal dummy variables are used.

Hamilton (my much dogged-eared version for Stata 7) explains how
to use _S._ in Stata, but not the "why," or the "when" to use it.
Harvey explains briefly both procedures and says that the
differencing is used when the seasonal pattern changes over time.
Thus, in this sense, the differencing would cause the seasonal
pattern to become stationary.

Is there ever an appropriate situation to use the _S._
time-series
operator outside of an ARIMA model? In answering this specific
explanation, and possible reference that I could research, in
controlling seasons (and cycles) in time-series data that goes
beyond the usual econometric textbook treatment. An example,
which
does not necessarily need to be economic, would be very much
appreciated.

Thanks,

Yvonne

M. Yvonne Reinertson
University of Florida, PhD Candidate
myvonne@ufl.edu

Version: Stata 7 (final update)
OS: WindowsXP

References Used:
Hamilton, L., 2003, "Statistics with Stata," (Duxbury-Thomson
Learning, Belmont).
Harvey, A, 1999, "The Econometric Analysis of Time Series," 2nd
ed., (MIT Press, Cambridge).

- --
REINERTSON,M YVONNE

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