Another way to say it: if we had any way to quantitatively measure the performance of programmers over time, most "programmers" wouldn't be programmers any more.

There's a field with a similar problem to our own: public-school teaching. Teacher performance isn't quantified, and so teachers can pretty much get away with only being as good at imparting knowledge as any random member of the population--even though they took years of education-in-Education. Most "teachers" aren't teachers, any more than I'm a teacher; they're simply people paid to repeatedly attempt (and fail) to be teachers.†

We pay a lot of people to repeatedly attempt (and fail) to be programmers.

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† Teachers' unions are right now fighting the introduction of actual metrics on how a student's achievement level's year-over-year velocity is affected by a teacher relative to the average of their peer-teachers in the same school. They are fighting this because the data clearly shows a bimodal distribution, where a lot of people are just extremely unfit to be teachers--their students actually reaching negative learning velocities by their presence in the class--and until now, the unions protected these people, since there was nothing to prove they sucked so very much. It's hard to go back on your decision to defend someone when you later find out that they're indefensible.