Published Review of Economic Studies Online 30 Sep 2025 Published Jul 2026The Surrogate Index: Combining Short-Term Proxies to Estimate Long-Term Treatment Effects More Rapidly and PreciselySusan Athey · Raj Chetty · Guido W Imbens · Hyunseung KangThis paper addresses a fundamental challenge in program evaluation: primary outcomes of interest — such as lifetime earnings or long-term employment — are often observed only with lengthy delays, …