Two significant barriers prevent comprehensive reference availability through Crossref.
The first barrier
First, two-thirds of Crossref’s publisher-members, in particular the smaller ones, do not submit references along with the other details of their publications. Many of these published works are of types (e.g. abstracts, editorials and news items) that lack any references. However, while the number of non-submitted references associated with other publications from these publishers is not known, it is likely to be substantial.
Ironically, quite a number of publishers have their Crossref reference status option set to ‘Open’, and yet fail to submit any references!
All publishers who use Crossref DOIs and submit metadata describing their works to Crossref should be strongly encouraged to start submitting associated reference lists if these exist. Crossref have confirmed that it is easy to do, with or without membership of Crossref’s free and beneficial Cited-by Service that provides publishers with statistics on the citations of their own publications. Help can be provided by Crossref Support (firstname.lastname@example.org).
The second barrier
The second barrier to full reference availability is created by publishers that submit references to Crossref, but do not presently make them open. Elsevier is by far the largest member of this group, which also includes the American Chemical Society, IEEE and Wolters Kluwer Health.
It is both quick and easy for a publisher to change its preference setting and request that all the references associated with its DOI prefixes are made open – all it requires is an email request to email@example.com. But without such a request, the references will be remain in the default ‘Limited’ status.
References that are not associated with Crossref
There are, of course, many scholarly publications, for example preprints in repositories such as arXiv, and journal articles and monographs from small academic publishers in the Humanities, that do not have Digital Object Identifiers issued by Crossref. There are also an increasing number of datasets in repositories such as Dryad that have associated references to the scholarly literature, but whose DOIs are issued by DataCite. None of these submit references to Crossref where they can be made available via the Crossref API, and separate additional measures will be required to capture and share their references with the community.