Hello :-)

I have two large panel data sets of companies that are listed in Europe and the US in the years from 2015-2019, the post phase starts in 2018. I start with the European data, which should be my treated sample. Unfortunately, some companies are not observable for each time points, thus my sample is unbalanced. So, here are my questions and I really hope, that you can help me to get familiar with this analysis:
  1. Am I right and each company in the control and treatment group should be at least two times in the respective groups: Once in the pre- and once in the post analysis period? Thus, I should drop observations from companies that are either only available in the pre-time or in the post-time, right?
  2. Is it a problem that my sample is unbalanced and that for some companies I have time series observations for all of the five years, whereby some companies are only available for two years. This “phenomenon” seems to be random. Can I leave the data like they are for analysis? Is there something I must change when doing a diff in diff with unbalanced data?
  3. I have to analyse the effect of a firm disclosure rule, which is effective for certain companies for reports issued from 2018 onwards. However, some European companies will not be affected. So, is it right to include only those companies in the European treatment group, which are mandated by law to provide these reports in 2018 and/or 2019? Or should I include all European companies in my treatment group, no matter whether they are affected or not?
I would be happy if you would help me.

Kind regards,
Linda