COUNTER is a standard set of metrics to report on usage of electronic content.  The latest release is 5.1.  COUNTER metrics are applied to usage, search and denials.  

Usage of content, such as articles or e-books, is recorded at the platform, title and item level.  For example, articles in an issue of a journal are counted at the item level, while the journal itself is counted at the title level.  The platform on which the content is hosted, is counted at the platform level.

When a user Investigates content, both the total and the unique number of investigations are recorded.  Requests for the full-text are recorded as investigations but are also reported separately

Searches are recorded when a database has been specifically requested, when searched with other databases, or when searched via an API.

Denials used to be known as turnaways.  They are known in COUNTER 5.0 and above as Access denied. This can be due to not having a licence to access the content or because the simultaneous user limit has been reached.

For more information, read the Friendly Guide to release 5 for librarians.

What 's new in COUNTER 5.1?

1. Book usage is reported using both Item and Title metrics. Title metrics are not affected.  New item-level reporting allows publish to report on chapter usage.  If applied, you'd be able to see which chapters in a book are being most used.
2. Data types have been expanded to enable publishers to report usage in a more granular way.
3. Access types now include:
   • Controlled (subscribed content)
   • Open (open access regardless of type)
   • Free to read (content made freely available for a time-limited period)
   • Free to read and open access types are removed from Journal Requests, Excluding OA Gold and Book Requests, Excluding OA Gold. These are relabelled Journal Requests (Controlled) and Book Requests (Controlled).
4. Book usage can now be reported more granularly at chapter level.  Unique Item requests would treat all chapter accesses and 1 unique item request.  In 5.1 each chapter would be counted separately.  If a user accessed 5 different chapters, the unique item requests would now show 5. 
5. There have been changes to Sushi metrics and the JSON schema.

For more information about these changes, read the Friendly Guide to Changes in Release 5.1.