Basically, it uses men who were eligible for the Vietnam draft (who enrolled in college in greater numbers than they otherwise would have, to avoid military service) to demonstrate that success from college graduates is more than a "these people are the ones who were already going to succeed" thing.
In programming, we'll always have (in any month) some % of workers moving from one job to another (either because they quit, the company they worked for went out of business, or they were laid off - because the company performed badly or their own poor or poorly perceived performance.)
And, in programming, there will always be some set of people who find themselves out of job because the company they were working for no longer needs their Delphi or COBOL or other outdated skill (not to say there are not jobs for those Delphi or COBOL programmers, but there are less and less of them, on average.) Those programmers have to get re-trained, etc.
These two groups make up that 4% - which is around the natural unemployment rate for the economy as a whole on average over the last 70 years (it's around 4-5).
If the unemployment rate was above this (basically at the national avg of close to 9) for programmers, then the difference would be cyclical - missing jobs based on the recession. Because it is so low, I;m wondering if it's we can conclude that programming (in today's economy, at least) is recession-proof.
Inflation is 3-4%.
I would have guessed that tech unemployment would be lower than the national average, but 4% is really low.