So what’s a developer’s output (also, over time, that’s a key)? Also bear in mind that a metric needs to be comparable across the field. You need to be able to compare two developers by their productivity across the field. Also, it’s not the “business value” of the work, that’s a metric of the work. A factory as long as it produces nails, measures productivity of the number of nails over time (not the value of said nails, their quality, etc). That’s comparable across factories that produce nails (which is the output in this case)