Beginning with the 2005–06 report, ARL is using an Expenditures-Focused Index instead of its traditional Membership Criteria Index in public ranking reports. The Chronicle of Higher Education has published the "Index of Expenditures at University Research Libraries, 2005-6" (requires subscription).
Here's an excerpt from the ARL Index page:
Starting with 2005–06 data, ARL is calculating an Expenditures-Focused Index as an alternative to the ARL Membership Criteria Index. The Expenditures-Focused Index replaces the public availability of the ARL Membership Criteria Index. The Expenditures-Focused Index is highly correlated with the Membership Criteria Index and less affected by changes in the collections variables. The methodology behind this new index is described by Bruce Thompson in his October 2006 paper, "Some Alternative Quantitative Library Activity Descriptions/Statistics that Supplement the ARL Logarithmic Index."