Trainable skills were the currency of the 20th century.
Before a repetitive task is automated, people can solve the same problem over and over, and get paid in proportion to how hard they are to replace.
Automation threatens to erode these position, but despite this, some skills remain fundamental to all others.
Namely: communication and first-principles decision making.
Unfortunately, it's very difficult to study these in isolation (without getting bogged down in a bunch of pseudo-work).
But with the right heuristics, we can get to the heart of the issue.