Getting the numbers in the first place is one challenge, but if you do metrics and measurement, you get hundreds of numbers and improvements, different each week. It's not like you're working on a single goal and target, and then wave goodbye when you're done. Who remembers all those numbers and improvements? What did they come down to cumulatively over the years? No one can know.
But crucially, one also works in teams. There's no way one person can claim credit on one measured improvement unless it's something very trivial and isolated, a single-person project.
There are many people saying that they read negative things between the lines about people who don't put quantifiable metrics to their CV items. I don't mind missing metrics, it's much better than having some fudged up bullshit metrics in there.
The CV is there to establish trust between two people who don't know each other. Don't fill it up with stuff which shines with fabrication and misrepresentation.
