For titles that are considered low-risk (e.g. widely held electronic serials with perpetual access backfiles), we generally retain one copy within SPAN. For titles that are considered medium- or high-risk, we try to retain two or more copies. Many materials, however, are only held at one SPAN library. When materials are held at more than one library, we generally choose the library whose run is most extensive to be the Archive Holder.
There is no single mechanism to determine the optimal number of copies to retain within SPAN. For each phase, we try to balance the relative risk to the materials with the number of potential Archive holders.