Following the first code-steps in Gretl provided by Professor Schreiber and a SAS code provided by Professor Angrist (again, many thanks to both Professors for your attention), I have performed a solution in R (sorry, at the moment I'm more familiar with this language) to this issue.

The code is weird, but is written with the aim to show the step-by-step process and check the intermediate results.

If someone is interested in this topic, the data and my code could be downloaded from 

Please, be sure that any error is mine. I would be grateful for suggestions, improvements and corrections.

Thanks a lot for your attention and best regards

José Perles

University of Alicante