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


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: RE: estimation with a time trend.
Date   Mon, 5 Jul 2010 13:36:05 +0100

I just meant some convenient middle. In fact, it needn't absolutely be 
within the range of the data. So, the word 'midpoint' is indeed not the
best one. 'Reference year' would be a better term if one were needed. 

FWIW, I strongly support choosing some reference date that is simple
and/or makes scientific or substantive sense. That is important for not
only interpretability within a study but also interpretability across
studies. 

There is no real virtue to using a mean or median for this purpose,
whatever their merits elsewhere. 

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

Martin Weiss

Then I do not understand the word "midpoint". I thought you meant
something
like the mean or median?

Nick Cox
Betreff: RE: st: RE: RE: estimation with a time trend.

Sorry; I meant 

year- (or -year- MINUS midpoint for interpretability) 

For example, if you work with (year - 2000) then intercepts have a clear
interpretation as "predicted value in 2000".  

natasha agarwal

On Mon, Jul 5, 2010 at 11:03 AM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
> As -egen, group()- maps to 1 ... whatever regardless of gaps, this
might
> in fact make things worse. What's wrong with -year- (or -year- MINUS
> midpoint) for interpretability?

I am afraid I did not understand "year- (or -year- MINUS> midpoint)
for interpretability?"

Martin Weiss

> Your code maps years in your dataset to integer numbers, and I am not
> sure
> that it gets you any closer to your final destination. So far, it has
> not
> hurt either...
>
> natasha agarwal
>
> I was trying to estimate a production function with an unbalanced
> firm-year panel data and wanted to include a time trend. However I was
> not sure if the time trend was created correctly.
>
> egen t=group(year)
>
> I was wondering if anyone could please tell me if this was correct?


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