Hi all!

I am working with the dataset in the attachment.
The data in the variable appln_id are American patents that are classified as green.
Therefor the variable Green_patent indicates a 1.
gvkey is the unique company identifier.

The data is from 2009, 2010, 2011, 2012 and 2013.

Now I would like to know how many Green patents a company has cumulative per year.
So in 2009 only the number of 2009 but in 2010 the number of 2009 and 2010 and so on.

Do you know how I could approach this?

Thank you so much in advance!

Kind Regards,

Jos van WeerenArray