Dear stata users, per year quarter I've information on origin and destination for the airlines in the US. I want to generate a new variable (frequency) that counts the number of destinations from (for example ATL) from a specific airline per quarter of the year.

I was thinking about using the egen command or collapse and than merge both datasets.

Create number of destinations from the airports
egen numdest = group(dest)

or:
collapse numdest, by(origin year quarter)
merge one to one

It looks like this: T100 domestic data
Year Quarter Origin Dest Airline
2005 Q1 ATL DWT Delta
2007 Q2 ATL MEM Continental