I have this problem:

I am investigating which factors that effect abnormal return in football transfers. I have a clubID and year, so panel data should be perfect for this. The problem is that I have multiple observation for the same Club for each year, why I cannot use xtset ClubID Year.

Instead I can run a standard OLS and then run the regression: reg CAR1 .. i.Year i.ClubID -> but is that wrong?

Because I also know that it is possible to xtset ClubID, and then run the regression: xtreg CAR1 .. i.Year -> is that more correct than the normal OLS?

Or is there another model that is better to use? It is because I also include the person that has executed the transfer. So that person's characteristics are the same within the year for the club. This results in VERY much serial correlation, also because the firms control variable as Cash Flow, ROA and so on is included so that it will remain the same value for all the transfers for the specific club within that year. So my data is full of serial correlation. How do I cope with that, and is it possible to do?

Br. Jonas