Brain Drain

"Brain Drain" has been one of the issues debated where development is concern. To my understanding, it is the situation whereby talents in developing countries are attracted to migrate and work in developed countries. This migration of talent is therefore known as "brain drain" (I might be wrong with this definition).

How do we measure "brain drain"?
I did a search on the articles regarding this topic and surprisingly, almost all of it was written with plenty of assumptions and uses mathematical models to predict the effect. I could not find any solid quantitative data showing "brain drain" and therefore it remains a question about the seriousness of this problem.

Is this "brain drain"?
To my understanding, "brain drain" is a flow of trained talents migrating from developing to developed countries. What if the potential talents have been attracted to these developed countries before receiving the full training? (i.e. brilliant students given scholarship to study in developed countries) Does it still consider "brain drain"?

Just putting down my thoughts regarding this issue. It seems to me that this is another difficult areas that quantitative information is unavailable. I guess developing countries might not be able to identify the amount of "brain drain". In a hypothetical situation, a person would not inform his/her country when he/she intends to move overseas. Therefore I was just wondering how do the country actually know the actual amount of "brain drain".