RE: st: How to add restriction condition in Stata

 From "FEIVESON, ALAN H. (AL) (JSC-SK) (NASA)" <[email protected]> To "'[email protected]'" <[email protected]> Subject RE: st: How to add restriction condition in Stata Date Fri, 29 Jul 2005 12:07:46 -0500

```I still don't see why you can't sum the ages. What's the difference between:

y ~ Poisson(la); log la = b0 + b1*age1 + b2*age2  with b1=b2

and

y ~ Poisson(la); log la = b0 + b1*(age1+age2) ?

Al Feiveson

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Raymond
Sent: Friday, July 29, 2005 11:05 AM
To: [email protected]
Subject: Re: st: How to add restriction condition in Stata

Thanks!
Actually, I'm using an age-period-cohort model, and I need to put all the
three variables in the model. Without restrictions, there will be an
identification problem since cohort=period-age.
Thanks for your suggestions. I'll try it.

On 7/29/05, Richard Williams <[email protected]> wrote:
> At 11:51 AM 7/29/2005 -0400, Raymond wrote:
> >Thanks!
> >But the reason I make coeffients of age1 and age2 be equal is to
> >solve an identification problem. So, I cann't just sum them up.
>
> I'm not sure how you are getting an identification problem in a
> poisson regression or how setting the effects for 2 dummies equal
> solves it.  Maybe you should write out your model.  But in any event,
> I think the test command I gave before will still work; and the
> poisson command does support constraints, so you could do
>
> constraint 1 age1=age2
> poisson y age1 age2, constraints(1)
>
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>

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```