st: Restricting sample size

[Amal Khanolkar <Amal.Khanolkar@ki.se>]

Hello All,

I'm running a set of multivariate linear regression analysis stratified by ethnicity (ethnicity_bi2x) as follows:

eststo clear
eststo: regress syst bwtgestage_sd age byear i.conscript_office bmi height_cons ib2.education2 ib3.gestwk_catx2 if multibirth==1 & sex==1 & ethnicity_bi2x==1, cluster (mor_lopnr)
eststo: regress syst bwtgestage_sd age byear i.conscript_office bmi height_cons ib2.education2 ib3.gestwk_catx2 if multibirth==1 & sex==1 & ethnicity_bi2x==2, cluster (mor_lopnr)
eststo: regress syst bwtgestage_sd age byear i.conscript_office bmi height_cons ib2.education2 ib3.gestwk_catx2 if multibirth==1 & sex==1 & ethnicity_bi2x==3, cluster (mor_lopnr)
esttab, ci 

The ethncity_bi2x variable has three categories as follows:

ethnicity |
     cauca & |
   non-cauca |      Freq.     Percent        Cum.
-------------+-----------------------------------
     Swedish |    394,050       96.86       96.86
    European |     10,287        2.53       99.39
non-European |      2,496        0.61      100.00
-------------+-----------------------------------
       Total |    406,833      100.00


The Swedish group is very large relative to the two other categories. As this might bias results for potential differences in outcomes of interest between the three ethnic groups (non-Europeans being very much smaller then the Swedish group), I would also like to run the analysis by reducing the Swedish group from 394,050 to say just 50,000 or 100,000 subjects. How would I tell Stata to run the above analysis restricting to a random sample when ethncity_bi2x==1  (i.e. the Swedish group) ?


My other question would then be how would I choose an appropriate number for restricting the sample for the Swedish group? 

Thanks for any help in advance

Kind regards,


/Amal 



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