We evaluate journal cost effectiveness using an advanced version of Cost Per Use (CPU).
While we normally refer to our version as CPU in the application, its technical term is Net Cost per Paid Use, or NCPPU.
NCPPU is based on traditional Cost Per Use (CPU), but enhanced in what we consider some very important ways. We'll describe the differences here by breaking apart the phrase, starting with the end first:
Use is traditionally considered just downloads. This is great, but usage should also include other kinds of value, like citations to the journal and authorships in the journal. NCPPU includes this kind of usage.
Don't pay for free! When articles are available for free, usage of those articles shouldn't be considered a cost. Therefore NCPPU excludes all Usage to free content, such as articles available as Open Access or your Backfile.
The cost in NCPPU isn't just the subscription cost, because the alternative to subscribing doesn't cost $0. When libraries don't subscribe to a journal they still offer access to the journal via ILL, and this has a cost for the library. So we consider the cost to be the difference between the projected subscription cost and the projected ILL cost, were the library to not subscribe.
So that's what NCPPU is, and how we calculate Cost Per Use in Unsub! The net cost of subscription minus ILL, divided by weighted usage to articles which aren't available for free.
It's fun to note that this CPU can actually be negative! If the subscription cost is relatively low, and the Usage is relatively high, the estimated cost of ILL can be higher than the cost of subscribing to a journal!