Hi!

I have just appended three datasets from 2010, 2011 and 2012. They have many identical variables, but the 2011 dataset has some new variables that aren't in the 2010 dataset, and the 2012 dataset has some new variables that aren't in either the 2010 or 2011 dataset.

In the new dataset, I therefore have lots of missing observations for the new variables that aren't available in 2010. How can I drop all these new variables and keep only the ones that are observed in all years?

Note: there are roughly 250 new variables in 2011 and 200 new variables in 2012, so it would be tedious to drop them manually.

Thank you!