Re: AW: st: RE: RE: estimation with a time trend.

[Maarten buis <maartenbuis@yahoo.co.uk>]

--- On Mon, 5/7/10, Martin Weiss wrote:
> Then I do not understand the word "midpoint". I thought you
> meant something like the mean or median?

You want the value 0 to represent some meaningful point in time.
Most important is that it lies within the range of your data. 
Which point you choose after that is more a matter of convenience:

If you choose the minimum then your baseline represents the
begining of your observed period, which is sometimes helpful
when writing up your results. ("In the begining we found XXX, and
as time progressed ...")

It is often nicer to think in "nice round numbers" (e.g. multiples
of 10), so you can move this baseline a bit to the right by 
choosing "the next round number" (e.g. choose 1910 instead of 1907).

Data often tends to be sparse at the extreme ends of time, so
the minimum does represent a number within the observed period,
but the information about this period is often still very sparse.
Choosing some central value (mean, median), can for that reason
help the stability of some estimates in complex models.

You might be interested in one specific point in time, in which 
case it makes sense to declare that point as 0.

-- Maarten

--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany

http://www.maartenbuis.nl
--------------------------


      

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