use http://www.ats.ucla.edu/stat/hlm/faq/eg3all, clear unique childid /*number of students*/ unique schoolid /*number of schools*/ unique grade /*number of possible time points*/ preserve /*extract level-3 (school level) data*/ drop childid female black hispanic year grade math retained duplicates drop /*dropping duplicates*/ sort schoolid save eg3all_level3 restore preserve /*extracting level-2 (student level) data*/ drop year grade math retained duplicates drop sort schoolid childid count save eg3all_level2 restore preserve /*extracting level-1 (time level) data*/ drop schoolid female black hispanic size lowinc mobility sort childid save eg3all_level1 restore