[zaphods-towel]

I agree, this isn't really a measure of "brain drain." Brain drain is when educated people leave an area (usually a country) en masse. For example, one of my professors is from Sri Lanka, and she used her graduating class as an example of brain drain— more than half of them left Sri Lanka upon receiving their undergraduate degree.

[jwatte]

When that happens, ratio of educated workforce goes down. If we assume initial conditions for education are approximately equal, then outcome ratios do measure (the result of) brain drain.