Dear all,
I am working on a study about estimate the effect of policy implementation. Now I have data from 2000 to 2005, and the policy was promoted year by year from 2003 to 2005, firstly I used the PSM method to match the companies that received the policy in 2003, 2004, and 2005.

How can I continue to use DID method?

​​​Does the data of the years before the policy implementation (2000-2002) need to be matched year by year?

If the data before the implementation of the policy does not need to be matched, should it be appended directly?

Thanks.