Another possibility, Raoul,
Is to think of multiple imputation. If you think it a safe bet that the second child came at least nine months after the first and the third came at least nine months after the second, then that gives you a window within which you could impute a birthday for the second child--possibly using a uniform distribution within this window, or if you have some other information, incorporate that. This is a much simpler solution, although more (computer) time consuming. If you do this, be careful you use the correct standard errors at the end.
m.p.
>>> "raoul reulen" <r.c.reulen@gmail.com> 3/28/2007 8:03 AM >>>
Thanks Svend,
Could I also set the period between the first birth and the third to
missing? So I would know that up to the first birth (for which the dob
is known) the subject didn't have any children. Then set the period
between the first and the third birth to missing because I don't know
whether they had one or two children in that period
and then when the third child is born set it to 3 (so I would stsplit twice)?
period up to first birth: child_num=0
period between first birth and third child_num=.
period after third birth: child_num=3
By doing this the subject will at least contribute some person-years
up to the first birth and after the third birth. I guess I could also
add 9 months to the first birth, because I'm pretty sure that the
subject didn't give birth in that period. However, I 'm not sure
whether the above method is ok (though It would make it more complex).
Thanks
Raoul
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