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Re: st: Count variables and growth curves


From   Brendan Churchill <Brendan.Churchill@utas.edu.au>
To   "<statalist@hsphsun2.harvard.edu>" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Count variables and growth curves
Date   Thu, 28 Jun 2012 11:08:46 +0000

again thank you for your advice. 

I will check tomorrow morning - I have left the office for the evening.

Before I left, I used the xi:/.i command and ran each cohort grouping as a dummy and the constant looked fine so I will double check tomorrow

thank you!!!!!!

On 28/06/2012, at 8:43 PM, Maarten Buis wrote:

> On Thu, Jun 28, 2012 at 12:05 PM, Brendan Churchill wrote:
>> I am grouping year of birth into small five-year birth cohorts. I have year of birth from 1909 to 1986. When I recode this into 14 birth cohorts (for example 1981-1986 = 14). These have been coded from 0 to 14. I have also tried 1 to 15, but still the erroneous constant problem remains.
> 
> I have two guesses:
> 
> 1) You made an error when coding this variable and the new variable
> does not go from 0-14 or 15, but there are still some outliers
> present. This will particularly be important when you incorrectly
> added the new cohort variable as a continuous variable. Such
> coarsening only makes (some very small amount of) sense when you added
> them as a categorical variable (i.e. use the factor variable notation,
> see -help fvvarlist-)
> 
> 2) You correctly added this as a categorical variable, but the
> reference category is just too small to obtain reliable estimates of
> the expected outcome. You will see that the confidence interval around
> the constant will be huge.
> 
> I would typically prefer not to coarsen birthyear, but instead use
> splines to add a flexible effect of year of birth. See -help
> mkspline-.
> 
> Hope this helps,
> Maarten
> 
> --------------------------
> Maarten L. Buis
> Institut fuer Soziologie
> Universitaet Tuebingen
> Wilhelmstrasse 36
> 72074 Tuebingen
> Germany
> 
> 
> http://www.maartenbuis.nl
> --------------------------
> 
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