CRL conducted its audit with reference to :

• generally accepted best practices in the management of digital systems
• the interests of the CRL community of research libraries
• the practices and needs of scholarly researchers in the humanities, sciences, and social sciences in the United States and Canada.
• the criteria included in Trustworthy Repositories Audit & Certification: Criteria and Checklist (TRAC) 
• the Open Archive Information System reference model (OAIS)
• other metrics developed by CRL through its analyses of digital repositories

CRL assigned to Portico a level of certification in each of three categories. The numeric rating is based on a scale of 1 through 5, with 5 being the highest level, and 1 being the minimum certifiable level.

The general metrics used by CRL in assessments are based on the Trustworthy Repositories Audit and Certification checklist, and on other metrics developed by CRL through its analyses of digital repositories. TRAC was developed by a joint task force created by the Research Libraries Group (RLG) and the National Archives and Records Administration between 2003 and 2005, to provide criteria to be used in identifying digital repositories capable of reliably storing, migrating, and providing access to digital collections. TRAC represents best current practice and thinking about the organizational and technical infrastructure required to be considered trustworthy and worthy of certification.