# Re: st: age when having first child

 From "Austin Nichols" To statalist@hsphsun2.harvard.edu Subject Re: st: age when having first child Date Fri, 9 Jun 2006 09:10:05 -0400

```Dirk Nachbar --
Assuming you meant "derive the *dependent* variable from the adult's
and oldest child's age" i.e. you are regressing estimated age at first
birth on a set of independent variables, including cohort fixed
to look for as potential confounders (note also that education will be
endogenous).  Let me suggest another--if your survey does not include
children who have left the household, or includes them with lower
probability than children in the household, then older parents will
appear to have had children later.

Think of a pair of mothers who have children every 5 years beginning
at 25, where children are not observed after age 18.  A mom who is 44
will have 3 kids in the HH: the ones she had when she was 30, 35, and
40 (the one born when she was 25 is now 19 and gone from the data),
and her estimated age at first birth is 30.  A mom who is 42 will have
4 kids in the HH: the ones she had when she was 25, 30, 35, and 40
(the one born when she was 25 is now 17 and still in the data), and
her estimated age at first birth is 25. So the older cohort has a
later estimated age at first birth, with no real change.

On 09 Jun 2006 11:49:50 +0100, Dirk Nachbar <Dirk.Nachbar@dwp.gsi.gov.uk> wrote:
```
``` 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

WC2N 6HT London

020 796 28531

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