It looks like the BLS measures productivity as the dollar value of goods and services produced over the hours used to produce them.

Not much detail on those the dollar value is calculated, though.

Edit: I found a (somewhat patronizing) slideshow designed for K-12 students: https://www.bls.gov/k12/productivity-101

GDP divided by worker-hours, basically. It's a crude metric, but no one has ever managed to come up with a better one.

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