Identifying a common observation across the groups by year

[kamalesh pahurkar]

Hi, I have the following kind of data structure. I just want to keep or identify the product codes that are common across all the company codes by year.

year	companycode	itemcode
2002	11	115
2002	11	117
2002	11	119
2002	11	112
2002	12	117
2002	12	115
2002	12	119
2002	16	108
2002	16	117
2002	16	119
2002	16	111
2002	16	115
2002	16	120
2002	14	222
2002	14	117
2002	14	115
2002	14	119
2003	11	115
2003	11	317
2003	11	119
2003	11	112
2003	12	117
2003	12	115
2003	12	119
2003	16	108
2003	16	117
2003	16	119
2003	16	111
2003	16	115
2003	16	120
2003	14	222
2003	14	117
2003	14	111
2003	14	119
2004	11	115
2004	11	117
2004	11	119
2004	11	112
2004	12	117
2004	12	115
2004	12	119
2004	16	108
2004	16	117
2004	16	119
2004	16	111
2004	16	115
2004	16	120
2004	14	222
2004	14	117
2004	14	115
2004	14	119

[Clyde Schechter]

* Example generated by -dataex-. For more info, type help dataex
clear
input int year byte companycode int itemcode
2002 11 115
2002 11 117
2002 11 119
2002 11 112
2002 12 117
2002 12 115
2002 12 119
2002 16 108
2002 16 117
2002 16 119
2002 16 111
2002 16 115
2002 16 120
2002 14 222
2002 14 117
2002 14 115
2002 14 119
2003 11 115
2003 11 317
2003 11 119
2003 11 112
2003 12 117
2003 12 115
2003 12 119
2003 16 108
2003 16 117
2003 16 119
2003 16 111
2003 16 115
2003 16 120
2003 14 222
2003 14 117
2003 14 111
2003 14 119
2004 11 115
2004 11 117
2004 11 119
2004 11 112
2004 12 117
2004 12 115
2004 12 119
2004 16 108
2004 16 117
2004 16 119
2004 16 111
2004 16 115
2004 16 120
2004 14 222
2004 14 117
2004 14 115
2004 14 119
end

isid year companycode itemcode, sort

//    CALCULATE NUMBER OF COMPANIES IN DATA SET IN EACH YEAR
by year (companycode), sort: gen n_companies = sum(companycode != companycode[_n-1])
by year (companycode): replace n_companies = n_companies[_N]

//    KEEP ONLY ITEMS THAT ARE FOUND IN EVERY COMPANY FOR THE YEAR
by year itemcode (companycode), sort: keep if n_companies == _N

In the future, when showing data examples, please use the -dataex- command to do so, as I have here. If you are running version 18, 17, 16 or a fully updated version 15.1 or 14.2, -dataex- is already part of your official Stata installation. If not, run -ssc install dataex- to get it. Either way, run -help dataex- to read the simple instructions for using it. -dataex- will save you time; it is easier and quicker than typing out tables. It includes complete information about aspects of the data that are often critical to answering your question but cannot be seen from tabular displays or screenshots. It also makes it possible for those who want to help you to create a faithful representation of your example to try out their code, which in turn makes it more likely that their answer will actually work in your data.