How to drop observations?

[Yuchen Li]

Hello Statalist members,

I have a question about how to drop multiple observations from the same ID (or only keep one observation per ID). For example, my current data looks like this:

ID. Year
1 2010
1 2012
1 2014
2. 2008
3. 2011
3. 2014
4. 2007
4. 2009
4. 2012
5. 2007

I only want to keep the observation for the earliest Year (or drop the observations for the later Years) and I hope my new data would look like this:
ID. Year
1. 2010
2 2008
3 2011
4 2007
5 2007

Could you please let me know what code should I use?

Many thanks
Yuchen

[Clyde Schechter]

by ID (Year), sort: keep if _n == 1

In the future, when showing data examples, please use the -dataex- command to do so. If you are running version 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.

[Yuchen Li]

by ID (Year), sort: keep if _n == 1

In the future, when showing data examples, please use the -dataex- command to do so. If you are running version 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.

Will do. Thank you so much for your help Clyde!