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Re: st: age when having first child


From   "Emma Slaymaker" <Emma.Slaymaker@lshtm.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: age when having first child
Date   Fri, 09 Jun 2006 12:41:03 +0100

Hi Dirk,
You ask why you see an apparent decrease in age at first birth for
younger birth cohorts.  Did you use an ordinary linear regression to
model this?

If so, then only those people in the younger cohorts who have already
had a child can contribute to this regression model.  Many people in the
younger cohorts will be not have started their childbearing and those
that have will, by definition, have had their children at a young age.
The older cohorts will be nearer the end of their reproductive life when
surveyed & will include those people who had their children later in
life.  Excluding the young people who have not yet had a child biases
the data and can produce an effect like the one you have observed.  

To avoid this problem you need to carry out a survival analysis.  To do
this in Stata you need to -stset- the data, and then you can fit an
appropriate regression model or, if you simply want to look at change
over time, you could estimating the median age at first birth by birth
cohort.

Hope this helps,

Emma

>>> Dirk.Nachbar@dwp.gsi.gov.uk 09/06/06 11:49:50 >>>
All

I have survey data for one year from which I want to estimate an
equation for 
the age a person had his first child. I derive the independent variable
from 
the adult's and oldest child's age. When I regress that equation I have

significantly positive cohort effects for old cohorts which I find
quite 
puzzling for my UK sample. My expectation was that the age someone had
a child 
would increase over time. Do you have any views on what might have gone
wrong.

Dirk Nachbar
Assistant Economist
Pensim2
Department for Work and Pensions
Level 4, The Adelphi
1-11 John Adam St
WC2N 6HT London
020 796 28531
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