7.3 Counting Unique Items

Some COUNTER Metric_Types count the number of unique items that had a certain activity, such as a Unique_Item_Requests or Unique_Item_Investigations.

For the purpose of COUNTER metrics, an item is the typical unit of content being accessed by users, such as articles, book chapters, book segments, whole books (if delivered as a single file), and multimedia content. The item MUST be identified using the unique ID which identifies the work (e.g. chapter or article) regardless of format (e.g. PDF, HTML, or EPUB). If no item-level identifier is available, then use the item name in combination with the identifier of the parent item (i.e. the article title + ISSN of the journal, or chapter name + ISBN of the book).

The method for counting book usage in R5.1 at the item level is different than it was in R5. In R5.1, a Unique_Item_Investigation or Unique_Item_Request MUST be counted for each item (Book_Segment) that is used, independent of the method of content delivery.

  • Where Book_Segments can be identified within a Book, a Unique_Item_Investigation MUST be counted for each Book_Segment with which a user interacts and a Unique_Item_Request counted for each Book_Segment accessed in full. This includes where users download or view the whole book as a single file.

  • Where it is not possible to identify Book_Segments (e.g. the report provider does not have metadata identifying individual Book_Segments), the whole book MUST be counted as a single Book_Segment.

  • The same rules apply to identifying and counting usage of other items within aggregated works, such as Reference_Items within Reference_Works or News_Items within Newspaper_or_Newsletters.

To illustrate: PDFs for twelve chapters are downloaded within a single book in a single session. In R5.1 this must be reported as 12 Unique_Item_Requests and 1 Unique_Title_Request, where in R5 the same usage would be reported as 1 Unique_Item_Request and 1 Unique_Title_Request. This change has been introduced to allow more consistent reporting on the Book_Segment level in the Item Report and to facilitate accurate comparisons of usage across Data_Types, while retaining the ability to accurately compare book usage across platforms through the unique title metrics.

The rules for calculating the unique item counts are as follows:

If multiple transactions qualifying for the Metric_Type in question represent the same item and occur in the same user-sessions, only one unique activity MUST be counted for that item.

A user session is defined any of the following ways: by a logged session ID + transaction date, by a logged user ID (if users log in with personal accounts) + transaction date + hour of day (day is divided into 24 one-hour slices), by a logged user cookie + transaction date + hour of day, or by a combination of IP address + user agent + transaction date + hour of day.

To allow for simplicity in calculating session IDs, when a session ID is not explicitly tracked, the day will be divided into 24 one-hour slices and a surrogate session ID will be generated by combining the transaction date + hour time slice + one of the following: user ID, cookie ID, or IP address + user agent. For example, consider the following transaction:

  • Transaction date/time: 2024-06-15 13:35

  • IP address:

  • User agent: Mozilla/5.0

  • Generated session ID:|Mozilla/5.0|2024-06-15|13

The above replacement for a session ID does not provide an exact analogy to a session. However, statistical studies show that the result of using such a surrogate for a session ID results in unique counts are within 1-2 % of unique counts generated with actual sessions.