title: How to know how many countries are in my regression
body: <p>I am running a linear model regression with panel data (20 years, 150 countries) in r. I see that 2,000 observations are excluded from my regression due to missing value. Some 4,000 observations make it to the regression.</p>
  <p>I would like to know if any countries are excluded from the regression due to missing values. How could I find this out?</p>