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Re: st: Making Cohorts


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Making Cohorts
Date   Thu, 28 Oct 2010 11:04:03 +0100 (BST)

--- ajjee wrote:
> I have the following type of survey data
>
> Year: 1980 1981 1982
> 
> Countries: 1 2 3...........15
> 
> Waves 1980_1 1980_2 1981_1 1981_2 1982_1 1982_2
>
> variables: sex education age occupation .....
> 
> I want to make cohorts of this data for logit
> analysis. Please help me how to make cohorts.

I am assuming you want birth cohorts. The year
of birth is year in which the survey was held minus
the age of the respondent at that time, so:

gen cohort = year - age

As you can see above you cannot add cohort, year
and age to your model. They are a linear combination
of one another: if you know two you automatically 
know the third, so there is no additional information
present in the third. This means that you have to make
a decision which one of these three you choose to 
leave out of your model.

Typically it is a good idea to rescale such a 
variable to have the value 0 wihtin the observed
range and a more reasonable unit, e.g. decades
since 1950 instead of years since 0:

gen cohort2 = (cohort - 1950)/10

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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