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.
The methodology behind the Library Investment Index is described by Bruce Thompson in his October 2006 paper, "Some Alternative Quantitative Library Activity Descriptions/Statistics that Supplement the ARL Logarithmic Index" (PDF).
The principles and procedures for academic library membership in ARL are based partly on quantitative data that provide a view of the range of resources deployed among the existing members of the Association. To this end, ARL also calculates an annual ARL Membership Criteria Index for the university library members of the Association. The sole purpose of this index used to be a membership criterion for the Association; it currently serves as one indicator of potential for membership. The ARL Membership Criteria Index comprises the five quantitative data elements in which ARL university libraries most resemble one another. The Membership Criteria Index allowed ARL to evaluate potential new members and track the maintenance of membership requirements.
A detailed description of the ARL Membership Criteria Index is available in The ARL Library Index and Quantitative Relationships in the ARL, by Kendon Stubbs (Washington, DC: ARL, 1980).