Hi Forum,

I am struggling with something very silly but would be very helpful if someone could help me out.
I have an id variable which identifies the product and the size (e.g Coca Cola 600 ml)

Since my analysis is to evaluate a policy impact, I want to make sure that for the same product I have observations for both years.
However, there are some products that I have N observations for one year and 0 for the other, which could bias my research.

I am trying to drop those products who do not have observations for both years so I can compare the changes between them

Any suggestions?

Many thanks!!

Best,

Lena