Figure 1
Number of repositories using a particular type of persistent identifier. A total of 457 out of 1381 repositories (status of 14 Dec 2015) use some sort of persistent identifier. Some repositories use more than one type of persistent identifier.
Figure 2
Types of repositories using a particular type of persistent identifier. Note that some repositories use more than one type of persistent identifier and are defined as both disciplinary and institutional. “Other” PID seem to be important in disciplinary repositories, pointing to particular disciplinary groups of practice. Handle PID (non-DOI) are used by many institutional repositories, possibly due to use of Handle in repository software and early adoption.