Hello,

I am doing a difference in difference in panel data set, years 2001-2015. The observation units are US counties. The variables in my dataset are all industries (ex: fishing, manufacturing, tourism, etc...) measured both in terms of GDP by county and in terms of employment by counties (therefore, each industry enters in my dataset twice). For certain combinations of industry j and county i, I have missing values for all years;
Ex: Baldwin county may have no missing values for all industries except for fishing, for which there are missing values for all years. Similarly, Sussex county may have values for all industries except for mining extraction only (for all years), and so on and so forth.
The missing values are not random. Indeed they are suppressed data for matters of privacy (the values were generally small, so it was possible to date back to the firms).
I would like to ask:

1) how Stata deals, by default, with missing values when I run a regression;
2) how can I replace this missing value in the best and more reliable way.

I previously tried with "ipolate", but I read that the estimates would not be reliable


I hope someone can help me thank you in advance