st: Different results with the same code

[Estrella Gomez <estrellastata@gmail.com>]

Dear Statalisters

I am running a code and I have realized that every time I run it the
results (the coefficients and the standard errors) are different. This
is the code:

clear all
use "itunes_grav_song", clear
sort id country orig

egen  nb_id=tag(id)
bysort orig: egen nr_orig=sum(nb_id)
bysort orig country: egen nr_dest=sum(available)
gen avail_share=(nr_dest/nr_orig)
collapse (count) nr_songs=id (firstnm) contig comlang ldist
avail_share, by(country orig)
encode country, gen(d_id)
encode orig, gen(o_id)
gen home=country==orig

reg avail_share ldist comlang home i.d_id i.o_id


I have checked and after the collapse I obtain every time the same
dataset, so I don't know what can be the reason

Thanks for any help!
Estrella
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/