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
>>> [email protected] 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
**********************************************************************
This document is strictly confidential and is intended only for use by
the addressee.
If you are not the intended recipient, any disclosure, copying,
distribution or other
action taken in reliance of the information contained in this e-mail is
strictly prohibited.
Any views expressed by the sender of this message are not necessarily
those of the Department
for Work and Pensions.
If you have received this transmission in error, please use the reply
function to tell us
and then permanently delete what you have received.
Please note: Incoming and outgoing e-mail messages are routinely
monitored for compliance
with our policy on the use of electronic communications.
**********************************************************************
The original of this email was scanned for viruses by Government Secure
Intranet (GSi) virus scanning service supplied exclusively by Cable &
Wireless in partnership with MessageLabs.
On leaving the GSI this email was certified virus free.
The MessageLabs Anti Virus Service is the first managed service to
achieve the CSIA Claims Tested Mark (CCTM Certificate Number
2006/04/0007), the UK Government quality mark initiative for information
security products and services. For more information about this please
visit www.cctmark.gov.uk
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
