st: preserve-restore

[Grethe Søndergaard <dkstatstata@gmail.com>]

Dear Statalist

For some reason, the layout of the e-mail I sent to you a couple of
days ago was quite messy, since a lot of symbols had sneaked into it
after sending it. Therefore I am sending it again:

I have a couple of questions about the preserve-restore procedure and stset.

My dataset:
id father-id mother-id death var ...
1 1 10 0 1
2 1 10 1 1
3 1 20 1 1
4 1 20 0 1
5 2 10 1 1
6 2 10 0 1
7 3 30 0 1
8 3 30 1 1
...
save \Temp\hs.dta", replace


I want to compare all maternal half siblings within a family as well
as all paternal siblings within a family. In order to do this, I start
out by creating an id-variable for full siblings, paternal half
siblings or maternal half siblings and afterwards I run preserve-restore:

egen gruppe = group(father-id mother-id)
egen mgr = group(mother-id)
egen fgr = group(father-id)


forvalues x = 1/8{
preserve

*MATERNAL HALF SIBLINGS*
gen strata_mother = `x' if ((mgr==mgr[`x']) & gruppe != gruppe[`x']) |_n==`x'

*PATERNAL HALF SIBLINGS*
gen strata_father = `x' if ((fgr==fgr[`x']) & gruppe != gruppe[`x']) | _n==`x'

drop if strata_mother==. & strata_father==.

if `x' == 1 {
save " \Temp\hs.dta", replace
}
else {
append using " \Temp\hs.dta"
save " \Temp\hs.dta", replace
}
restore
}

I have the following questions:
1. Is there any way to make preserve-restore run faster (my dataset
contains more than 2 mil. observations so it takes about two days to
run it)
2. Is it problematic to create strata_father after creating
strata_mother in the same preserve-restore statement?
3. I want to use strata_father and strata_mother as strata
variables in a cox regression analysis - and I want to perform the
analyses separately for females and males. Since preserve-restore runs
slowly, I want to state this after having run it. However, it seems as
if it doesnt work to state that I only want to include e.g. males in
stset (if sex==M). Males who experience an event,
but who has no brothers but a half sister still counts as an
event. Is there any way to state in stset, that I only want to compare
males and that I only want to include events, if the male who
experience it has one or more brothers?

I hope this is clear but since I am not an experienced user of
Stata, please let me know if you need more details.

Thank you

Grethe
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