Driven by data limitations, many bank-level studies have resorted to the use of securitization proxies to measure securitization by banks. The diverging range of available proxies raises the question of whether all of them actually measure the same thing, namely securitization. We explore this question for a wide range of securitization proxies for large U.S. commercial banks (2011)(2012)(2013)(2014)(2015)(2016)(2017). We view securitization as an unobserved variable and use a form of Structural Equation Modeling to investigate whether there is a set of unobserved common securitization factors that explains the interrelationships among the securitization proxies. Our analysis identifies a single securitization factor. Eight out of eleven proxies measure this common factor significantly and reliably. The eight measures include some of the most widely used proxies in the literature. Our analysis provides a statistical rationale for their use as measures of securitization. 20 2. What Do Securitization Proxies Measure? 68 3. The Scale and Skill of Securitization