st: cox regression - half siblings

[Grethe Søndergaard <dkstatstata@gmail.com>]

Hello,

I have a dataset that contains information on a population born in
1980-1990. Some of the persons in this dataset are siblings (both half
siblings and full siblings). I want to perform cox regression on the
entire population as well as on populations of siblings, i.e:
- a population of individuals who are full siblings
- a population of individuals who are half siblings (same mother,
different fathers)
- a population of individuals who are half siblings (same father,
different mothers)

The dataset includes the following variables:
Person_ID
Mother_ID
Father_ID
Health indicators
Education

I am going to use stcox to analyse my data. When I do the analyses
based on siblings, I use "strata" in order to stratify the analysis on
siblings. When doing the analyses for full siblings I write:

stcox i.education, strata (mother_ID father_ID)

But what do I do, when performing the analyses on e.g. half siblings
with the same mother but different fathers? If I only stratify on
mother_ID I will include all children with the same mother, meaning
both full siblings and half siblings (with the same mother). So how is
it possible to perform this analysis on half siblings only?

Thank you!
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