Good Evening.

I am dealing with a project in Econometrics, in which I have to see what is the effect of plyaing football matches under close doors after the pandemic.

I have a panel which includes all the football matches of the Italian Second League played last year and this year.
The ultime goal would be to see if there is some difference due to the closed doors on the number of goals scored or in the difference between the number of goal scored and conceded in the Italian Serie B League.

I just have a very basic question, how do you treat some squads that "leave" the sample? For example, I got some squad that got promoted in the Italian Serie A. In this case, I think that this would not be a problem, since I have data from Semptber 2019 to August 2020, so I basically have data on both the pre and post covid period. On the other hand, there are some teams that got promoted from inferior leagues/relegated from Serie A. In this case, I do not have data on the "pre covid period" regarding those teams.

Would it be correct to simply remove these teams from the sample and to focus only on the teams that played both Leagues, that is, Serie B 2019-2020 and Serie B 2020-2021?