That page also includes studies that talk about course pass rates.
I don't know if our efficacy work necessarily changes "mastery" definitions, but I can imagine (but have no direct involvement with) these studies and smaller-scale studies are used by our pedagogy and course creators to improve how we approach the material.
Standardized testing is indeed one way that we can demonstrate efficacy:
https://www.khanacademy.org/about/impact
That page also includes studies that talk about course pass rates.
I don't know if our efficacy work necessarily changes "mastery" definitions, but I can imagine (but have no direct involvement with) these studies and smaller-scale studies are used by our pedagogy and course creators to improve how we approach the material.
Again, though, not my area of expertise :)