A "reliable proxy" means having a trustworthy substitute or representative. It refers to someone or something that can be trusted to act on behalf of another person or system accurately and dependably. Full definition
Further, as with program quality, the standard measures of teacher quality (degrees, experience) are not reliable proxies for what teachers do in the classroom. (educationnext.org)
A more reliable proxy might be the pace that can be sustained by breathing through the nose. (marksdailyapple.com)
Furthermore, test - based accountability is built on the assumption that test results are reliable proxies for success later in life, but research has called that assumption into question. (educationnext.org)