From the Association of Research Libraries Library Investment Index Web Page:
Each year, ARL calculates the ARL Library Investment Index (previously named Expenditures-Focused Index). Beginning with data for 2005–2006, this index replaces the public availability of the ARL Membership Criteria Index. The Library Investment Index is highly correlated with the Membership Criteria Index and less affected by changes in the collections variables. Both indexes are summary measures of relative size among the university library members of the Association. Neither index attempts to measure a library’s services, quality of collections, or success in meeting the needs of users. The Library Investment Index has been calculated retrospectively beginning with data from 2002–2003.