The findability and interoperability of some Permanent Identifiers (PIDs) in use on the internet and their compliance with the FAIR data principles are explored. It is suggested that the wide distribution and findability (e.g. by simple 'googling') on the internet may be more important for the usefulness of identifiers, than the resolvability of links by one single authority, purportedly guaranteeing their permanence and authenticity. The prevalence of phenomena such as link rot implies that the permanence of URLs, PURLs or URIs cannot be trusted. By contrast, the well distributed, but seldom directly resolvable ISBN identifier has proved remarkably resilient, with far-reaching persistence, inherent structural meaning and good validatability, by means of fixed string-length, pattern-recognition, restricted character set and check digit. Adding context and meaning to identifiers through namespace prefixes and object types is also suggested. Arguing for a wide distribution of validatable identifiers, the conclusion resembles the experience of the boy Marcus in the novel-based film About a boy, from living with a suicidal mother: It's not sufficient to rely on one source only for sustenance. You need more than that. You need backup, in case something happens.

Introduction: Identifiers in science

Identifiers in science may refer to digital or physical objects, or concepts. They may be general or domain-specific. Among the more prevalent general PID, permanent identifier types are DOI, Handle and UUID. There are also 'old', bibliographic identifiers like ISBN. Created in the 1960's and -70's of the print era, how come they survived into this digital age? Some reasons might be: they are well distributed across the internet, and widely used by stake-holders (libraries, publishers, readers). They have a semantic structure, identifying well-defined objects, and a fairly precise validation mechanism through fixed string-lengths, limited character-set and check digits. Some of these properties of good identifiers are shared by DOIs, Handles and UUIDs, or other more domain specific identifiers used for scholarly data, but seldom all of them simultaneously. Among these characteristics the main focus here will be on validation, as seemingly somewhat neglected lately.

Identifiers - why do we need them?

The general purpose of identifiers is to serve as references to the objects that they are supposed to identify. This requires identifiers to indicate, preferably in and by themselves, what type of objects they are meant to identify. Now, far from all identifiers fulfil this requirement. Rather, it is often left to the names of things to describe the objects identified, thereby providing context and meaning. Scientific names may or may not be part of the metadata on the destination page of a PID-URI.

While scientific names are often useful for describing objects (or at least certain aspects or properties of 'things', organisms), they have other drawbacks compared to identifiers, some of which were identified by . For example, homonymi and disambiguation should generally be a lesser problem for globally unique identifiers. And while concatenations or abbreviations may be problematic in the use of names for identification, string-length restrictions and pattern limits are useful for validation of identifiers, as is avoiding white space. Missing or added characters, all types of misspellings are easier to detect and validate in standardized identifiers of fixed string-length or well-defined character patterns. Inconsistent encoding should generally also not be a problem in good identifiers, for which the set of allowed characters may be limited. However, these assets of some identifiers may conflict with the legitimate interest in having also transparent, meaningful PIDs that at least in part "speak for themselves". The result of a compromise between these two interests may be seen in the Handle (hdl) system (below).

FAIR principles

The FAIR guiding principles aim to make data Findable, Accessible, Interoperable, and Re-usable. As such they concern also metadata in general and identifiers, PIDs, in particular, as is seen from some of the principles:

The FAIR data principles.

The FAIR data principles.

The FAIR principles clearly need interpretation to become fully operational, as several observers have noted, and such work is also in progress. Further explications of some of the principles are also available from the originators :

The FAIR principles have some resemblance with the Linked Data rules first fomulated by Tim Berners-Lee already back in 2006, updated in 2009 and 2010. There is, for example, the insistence on machine-readability and the linking of data sources with other data. But the FAIR principles do not necessarily require metadata to be expressed in RDF, as the Linked Data rules demand. It seems also that Berners-Lee is critical of identifier types using other schemes than simple HTTP URIs.

Neither the FAIR principles, nor the Linked Data rules say anything explicitly about validation. Still, particularly for the Interoperability and Re-usability, it is crucial that metadata can be properly validated against a schema, as adhering to an accepted metadata standard. And this includes also identifiers. We must be sure that they are of the type or format they claim to be, even if they cannot be resolved to a dedicated URI any longer. Failed validation, e.g. due to simple typos or wrong namespace, may even be one way of checking why an identifier or URI does not resolve as expected. It is also important for the possibility to export metadata to another format, thereby promoting the re-use of data, without exporting also potential errors. Although transformation or harvesting of metadata might be possible even without validation, the trust in the results and quality as well as the eventual findability of the data (and so again the re-usability) might be seriously affected. For enhanced findability, it is also important that standard, widely distributed identifiers are used.

Validation of an identifier means ensuring that it is true to its proclaimed type, for example, making sure that what is flagged as an ISBN is not in fact an ISSN (real use case), or that the string-length and check-sum is compliant with its type. A further advantage of promptly validatable identifiers, as against relying exclusively on resolvability, is that validation can be performed also off-line, by means of a more or less simple validation-algorithm and a piece of software such as a resonably good XML-editor.

Resolvable or findable?

In the present FAIR principles the focus is very much on resolvability of identifiers by an authority , despite the general awareness of phenomena like 'link rot' and 'reference rot'. It has even been suggested to put up digital gravestones over disappeared resources, with metadata from their last known whereabouts serving as epitaphs. A 2013 study in BMC Bioinformatics analyzed nearly 15,000 links in abstracts from Thomson Reuters’ Web of Science citation index and found that the median lifespan of web pages was 9.3 years, and just 62% were archived. This happens although there is an understanding that [u]nique identifiers, and metadata describing the data, and its disposition, should persist -- even beyond the lifespan of the data they describe. A recent study of some 40 research data repositories found that only one of these (3%) was compliant with the FAIR principle of Accessibility requiring a clear policy statement (or various examples of data this has actually happened to) indicating that metadata is still available even if the data is removed. The argument here, though, is not that resolvable, permanent URIs should be avoided as identifiers; in fact, they often do serve their purpose of providing a more persistent metadata source than "ordinary", plain URLs. But, as has been eloquently remarked, "persistent URIs must be used to be persistent". Persistent, resolvable URIs as identifiers work by means of a decoupling of the location and the identification functions of URIs.

The custodian of a web resource maintains the correspondence between the identifying URI and the locating URI in the resolver’s look-up table as the resource’s location changes over time. ... The solution comes at a price because it requires operating a resolver infrastructure and maintaining the look-up table that powers it.

This is true of DOIs, as well as Handles, PURLs and URNs. There are in fact numerous cases when the lookup-table is not maintained and updated as required. That is why it may be wise not to rely on a single 'custodian' for the resolution of identifiers and access to associated metadata. It is rather the distribution and use of identifiers - whether resolvable or not - that is important here. It seems not even the authors of are true to their own principles, since three of their references that actually have DOIs are cited without them: . So again, PIDs must be used and cited to remain permanent. Citations may also serve as a 'means of transportation' to achieve widest possible distribution. Further, it may be argued that wide distribution is dependent on good 'validatability', in order not to multiply errors and 'non-resolution' as a result.

One way to achieve wider distribution of identifiers might be the praiseworthy initiative of signposting.org (see also: ), using HTTP headers for retrieval of PIDs from simple URLs:

curl -I "https://doi.pangaea.de/10.1594/PANGAEA.867908" HTTP/1.1 200 OK Content-length: 8424 Content-type: text/html;charset=UTF-8 Link: <https://doi.org/10.1594/PANGAEA.867908> ; rel="identifier"

However, the signposting initiative, so far, only redirects the question of use from PIDs and DOIs to HTTP header links. Again, going back to the question of resolvability, the relationship between identifiers such as DOIs and URIs/IRIs is not always straightforward, and sometimes involves a chain of redirects ('303s'), before reaching eventually a destination holding also the appropriate metadata. . Another reason resolvability may not be sufficient, even if the metadata is somehow in place, is that the file on the destination page resolved to is behind a paywall. This is a recent case, where apparently public domain content more than 100 years old was hidden behind a DOI-resolver charging 50$ for release of the content:

Tweets about the use of DOIs referring to resources behind paywalls.

Tweets about public domain resources behind DOI paywalls.

Tweets about the use of DOIs referring to resources behind paywalls.

Tweets about public domain resources behind DOI paywalls.

First, it is not true that every object only gets one and only one DOI. It is possible to mint several DOIs for the same resource by different agents, such as Dataverse, Figshare, ResearchGate etc. The DOI in question here is: 10.1080/00222930908692639. Secondly, there are some remedies against these cases, notably the recently launched oaDOI at oadoi.org. We try it out here:

oadoi.org tool finding open access version of a resource referenced by a DOI behind a paywall

oadoi.org tool finding open access version of a resource referenced by a DOI behind a paywall.

In several steps this eventually leads us via an API to an XML-file with a link to the freely accessible fulltext at http://www.biodiversitylibrary.org/part/60220. But, most often you are not so fortunate to find a free replacement copy of resources behind a paywall. What meets you when tested in those cases may be this telling message, when tried for the DOI of :

oadoi.org tool finding open access version of a resource referenced by a DOI behind a paywall

oadoi.org tool not finding open access version of a resource referenced by a DOI behind a paywall.

And if oadoi.org (apparently just recently replaced by the browser add-on unpaywall.org) fails, identifiers.org SPARQL endpoint might be useful. But, it does not necessarily give us an open access URI in return. And it only works if the potential corresponding URIs have been assigned the property owl:sameAs just as the submitted subject URI. Unfortunately, in neither of our cases above these conditions are met.

Assuming we have finally found a single seemingly reliable custodian of our PIDs and URIs, promising 24/7 resolution and top quality metadata, should we rest content with that? Most serious lawyers and journalists probably would agree, it is wise not too judge by the testimony of a single witness, a single source alone. The evidence of at least two, mutually independent witnesses is generally preferred. Clark describes multiple resolution as representing a stage in the evolution of PIDs, that will eventually be surpassed by a more mature age when we supply also data types to come with the PIDs, in order to make them more machine actionable . Providing multiple access to, or identification of resources through PIDs, that are capable of serving as trustworthy, competent, valid independent witnesses from different moments in time, at different sites, in different places is a good idea. Thus, we accept that an object may have multiple PIDs. Ideally these multiple PIDs should get to "know about" each other as a way towards interoperability. . This can be achieved already, e.g. by means of linked open data (LOD), sameAs-relationships and tools provided by n2t.net, oadoi.org and the identifiers.org SPARQL endpoint referred to above. Multiple identifiers from different namespaces for the same object may even be desirable in order to ensure interoperability in different environments. . It is also in line with the principle of the semantic web known as the NUNA, Non-Unique Naming Assumption, implying that things described in RDF data can have more than one name and any object may be identified by more than one URI, serving in RDF as 'names' of things.

Cartoon of proliferating standards.

Proliferating standards cartoon. Source: xkcd.com. CC BY-NC 2.5

However, the conclusion to be drawn from this cartoon and the website yapid.org with regard to PIDs is not that any identifier is as good as the other. In fact, there are significant differences in quality between identifiers, particularly in terms of 'validatability' and 'meaningfulness', or 'semantic weight'. We are getting there a bit later.

But first, having referred to linked data and sameAs-relationships as a possible solution to achieving interoperability, what about long-term sustainability? Are LOD, relying heavily on URIs, fit for survival? Archival records for long-term preservation need to be self-sustained, carrying meaning within themselves, while the references may no longer be resolvable. In e-archives compliant with the OAIS-model and Trustworthy Digital Repositories standards for self-sustenance, this means that URIs lacking an inherently meaningful structure will often serve only as another set of dumb identifiers. Unless they can import some meaning from outside, through resolution or sameAs links, such opaque, non-resolvable URIs should henceforth rather be described as "non-semantic".

Which identifiers are FAIR enough?

We must ask about PIDs, Permanent Identifiers, just how permanent are they really? Even if not always resolvable, are they in general still 'findable', well distributed over the internet in time and space? Are they 'validatable' (e.g. through fixed string-length, pattern-recognition, restricted character set, built-in checksum, built-in type?) Are they FAIR?

Findability: Beginning with the F for findability, for comparison we go back in time to 'old-fashioned' ISBNs, Internaional Standard Book Numbers. Publicly declaring what type of objects they are meant to identify, ISBNs are rarely directly resolvable. But they are widely distributed, they have good findability in terms of precision hits, as seen by simple 'googling', with good survival rate, longer than the median age of web-pages 9.3 years. For example, look at ISBN 0-14-029161-X: The Diversity of Life / Edward O. Wilson (2001). Simple googling of 014029161X, unprefixed and without hyphens results in 57/57 precision hits (date: 2017-01-30). ISBNs could also be searched in library catalogs, the most comprehensive of which is probably the Karlsruhe Virtual Catalog – KVK worldwide. Result of the query '014029161X', with the same unprefixed ISBN without hyphens yields 123/123 precision hits, recall being difficult to compute since in 55 of 72 catalogs the search could not be successfully processed or no records were found. To counteract the possibly unfair bias with a modern classic like this, we try instead an even older, and presumably less well-known example: ISBN: 2130381030. L'Identité : séminaire interdisciplinaire dirigé par Claude Lévi-Strauss, 1974-1975 (Paris: PUF, 1983). Googling without prefix (2130381030) the precision is between 14/39 and 22/50; with prefix (ISBN2130381030) it reaches as high as 17/18 (date: 2017-01-30).

These results could be compared to similar tests for identifiers of one of the most well studied organisms of all, the fruitfly Drosophila melanogaster. Starting with its GBIF ID 5073713, googling the unprefixed pure number gives the modest precision of 1/107 (date: 2017-01-30). Using instead the Global Names Resolver to get a UUID v. 5 for Drosophila melanogaster, <gni-uuid>1bc2f359-47e4-5da6-a748-74676b7c8c5d</gni-uuid>, googling it either unprefixed or prefixed gives a zero result (0 recall, 0 precision, date: 2017-01-30). Trying instead the same UUID in a general search of all databases of NCBI, the US National Center for Biotechnology Information), we get 34 'hits' in 13 different databases, but all wrong (i.e. 0 precision) 1bc2f359-47e4-5da6-a748-74676b7c8c5d. Apparently this is because the default search algorithm ignores the hyphens, or rather replaces them with a 'OR', so that we get chunks of the string interpreted as e.g. part of a gene locus or names of clones. Most notably, we get 0 hits in the NCBI Taxonomy database, that on the face of it would seem to be the most relevant to our search. Going back instead to the GBIF, using our gni-uuid for an overall search, restricting our search in several steps finally to only species we still get a result of 120613 'hits', simply too much to make precision all but negligible. So, while the UUID is imminently validatable, with fixed string-length and restricted character set, it is neither directly resolvable, nor findable in terms of search results. To earn significance and importance as identifiers gni-UUIDs must become more findable and re-usable, for example by ping-back and auto-update, assigning themselves to the records in the biodiversity database sources they were drawn from and make use of schema.org and similar to get incoming links and a better ranking in search engines.

Accessibility: Data and (digital) objects are accessible only in so far as identifiers are findable or resolvable preferably to open access landing pages with either direct availability of resources, or sufficient metadata to direct the user to such an access point. In this respect DOIs are often, but not always, as good as or sometimes better than ISBNs (for obvious reasons regarding print only material), while gni-UUIDs as described above are all but useless.

Interoperability and Re-usability are both intimately associated with 'validatability', as argued above. We will look more into detail at the performance of different PIDs regarding this below.

DOI: DOIs can look just like anything. Here are some real cases, all at the time of writing resolvable and with multiple findability also by simple googling, some of them pretty 'old', although they got their DOIs assigned fairly recently. One is even from 1977 (doi: 10.1177/030631277700700112), but it still produces an impressive precision score of 26/26 or 59/59 (date: 2017-01-30), mostly due to it quite high citation rate, yielding hits for all the citing sources.

Now, following are some old DOIs from Wiley Online Library 1996,1998 and Springer 2001 that do not seem to resolve properly (on 2017-01-31):

Obviously, all these DOIs, whether resolvable or not, vary substantially in string-length, from just 17 to over 60 characters, some involving abbreviations of journals or organisations, one an ISBN, and some containing characters in need of special XML-encoding, different from URI. Note that although the two last items in the first group are from the same journal, Scientometrics, they are quite different in structure. Anyway, all the items in both groups are valid DOIs and all validate against the best we can offer as a schema, with only partial pattern recognition:

 <sch:rule context="identifier[@type='doi']">
   <sch:let name="doi-pattern" value="'^(doi:10|10)[.][0-9]{4,}/\S+$'"/>
   <sch:assert test="matches(., $doi-pattern, 'i')">All DOI identifiers must start with 10, followed by a minimum of 4 digits, a '/' and a suffix of any length

But, so does this fake DOI equally:


DOIs, as we have seen, unlike ISBNs are difficult to validate accurately. Or rather, it is difficult to find sufficiently discriminatory criteria to distinguish proper DOIs from fake ones. They have no fixed string-length, to start with, and very little of character set restrictions. All we can have is a partial recognition-pattern such as the one in the schematron rule above.

Handle:The Handle identifier system seems fairly easy and handy at a first glance. Only, it comes in two different flavors. One is the semantically opaque, which has the structure: Prefix/noid (10079/sqv9sf1), where the NOID-part (for Nice Opaque Identifier ) is a 7-character long alphanumeric string from the restricted character set "0123456789bcdfghjkmnpqrstvwxz", with random minting order. The other flavor is the semantically transparent, which could be of three different types: the URL handle: Prefix/local-PID (10079/bibid/123456), the user handle: Prefix/netid/netid (10079/netid/guoxinji) and the simpler group handle: Prefix/group (10079/ISPS). While being more instantly "meaningful", providing context, this kind of Handle, however, will prove less "validatable" in the sense that there is no longer any fixed string-length or restricted character-set.

UUID v5 has support within the field of biodiversity taxonomy, as an important complement to scientific names. . They were introduced to the field in 2015 by the Global Names Architecture - GNA . The arguments for using them instead of name strings for certain functions are that they save space as index keys in databases, they have a fixed string length (36 characters, including the dashes) while scientific names are of different length. UUIDs do not suffer, as names sometimes do, from encoding problems that are difficult to detect and they are more easily distinguishable one from the other than name strings for closely related species variants. Specifically, it is argued that UUIDs v5 ... can be generated independently by anybody and still be the same to the same name string... Same ID can be generated in any popular language following well-defined algorithm. The corresponding Ruby Gem app is described thus: Creates UUID version 5 out of scientific name string. It uses globalnames.org domain for DNS namespace. There is a 1:1 relationship between the string and the corresponding UUID, so it allows globally connect data about a name string originated from independent sources without a need to negotiate identifiers. Note, however, that it is actually the specificname string that is identified here, not the object, the organism, the 'thing itself'. Thus, the resulting UUID is completely dependent upon the particular name string (with its encoding), it cannot be used as a bridge between different name forms for the same organism, telling us that they are naming the same object. This is due to the fact that it is generated by hashing a namespace identifier and name. By contrast, the UUID v5 is easily validated, e.g. with an online validator.

Why context?

Generally speaking, although it is preferable that identifiers be findable and identifiable also in their unprefixed, pure form, typed identifiers give context by means of namespace prefixes of a metadata standard, a vocabulary or ontology. They tell us what kind of identifier it is and sometimes what kind of objects it is used for (e.g. ISBN), but not always (cf. DOI, EAN, UUID). Most importantly they indicate what schema, which rules should be used for their validation.

Page claimed that e.g. "dc:title" is adding "unnecessary complexity (why do we need to know that it's a "dc" title?)" in the JSON expression:

 { "@context": { "dc:title": "http://purl.org/dc/terms/title" },
                "dc:title": "Darwin Core: An Evolving Community-Developed Biodiversity Data
                Standard" }

A simple answer is that namespaces are important to retain meaning from context, serving as a key to interpretation for the future. Self-sustained long-term preservation should ideally mean in a case like this that the dc specification and schemas valid at the time be archived together with the records, or at least that there is provenance metadata including timestamps and namespace of terms used. Metadatafiles in XML usually have a xsi:schemaLocation indicating which schema to validate against, possibly also its @version. This information, together with timestamped metadata elements such as 'dateIssued' should be sufficient to provide context. For JSON metadata there are name/value pairs such as { "protocol": "doi", ... "createTime": "2017-01-12T10:49:03Z", ...} that could fill the same function. Secondly, context is just as important for validation of records also in the present.

A new contextual, integrated, validatable DOI - a BUOI?

As we have seen in the case of Handle above, validatability sometimes comes at a cost: transparency lost. Are we forced to make a choice between the two, then, and let identifiers be fully validatable while we let associated, linked scientific names stand for transparency and meaning? Or, can we create identifiers that are both fully validatable and at the same time more meaningful, providing context? So, here we finally suggest such a 'yapid' model for a 'BUOI' (Best Unique Object Identifier):

Model: [namespace prefix].[object type].[object id: 10 positions]_[version]_.[issued date:YYYY-MM-DD].[registrant: org.id/ORCID]
Example (expression of this paper): fabio.Preprint.philipson1_v1-1_.2017-03-25.0000-0001-5699-994X

It is a model of a structured, contextual, modular, validatabale identifier. To make it easier to implement, and more generazibale, there is no requirement of fixed string-length for the two first modules. This means already existing namespaces and object types could already be used to create a BUOI.

Each module/section may hold both letter characters and digits from a limited character set. The full stop (.) was chosen as module separator, since it works well in both xml- and http-environments, without encoding, and is not subject to confusion as sometimes hyphens and dashes (en-dash and em-dash) can be. It also works for tokenization of strings. The object type identified in the second module should belong to the initial namespace prefix. Every namespace can have as many object types as it likes. Namespace schemas could also define valid data types for their different object types, thus moving a step further towards supplying data types to come with the PIDs, in order to make them more machine actionable.

The scalability of the BUOI will mainly depend on the 10 character object id-module and how restricted the permitted character set is. A character set restricted to e.g. [A-Za-z0-9] will still have a possible of 6210 unique permutations, within each namespace and object-type, still better than the 7 character Handle with NOID. But if this will not be sufficient, the permitted character set will have to be expanded. It is also conceivable, to allow for integration of already existing identifier schemes, that a namespace sets its own character set and string-length restrictions, as long as these are declared in the validation schemas of that namespace or they have otherwise well-known validation algorithms. Now there are also narrow identifier namespaces that do not have as yet different object types defined, possibly since they comprise basically only one type of object. Such is the case basically for e.g. ISBNs and ISSNs. To allow also for these in the BUOI model, we suggest as default second module 'NOT' = No Object Type. So we could have BUOIs expressing e.g. an IGSN, International Geo Sample Number :

Example: IGSN.NOT.IECUR0002.2005-03-31.gswa-library 

The identifier should thus be fully validatable as a whole or in part (modules) in the corresponding namespace(s). Possibly the version and last two modules might be optional, but they are meant to offer built in data provenance. For organisation identifiers (org.ids), we are still awaiting a common standard like the ORCID for persons. Thus, the BUOI identifier should be right-truncatable so that the same object id from different dates and registrants could easily be searched for.

The resulting BUOIS should be minted within the corresponding namespaces, who would also be the 'custodians' and resolving authorities of their BUOIS, responsible for their uniqueness within their namespace. Another task would be to monitor and assign sameAs-properties to these BUOIs when identical twins of the same 'thing' are detected in other namespaces.

It has been suggested that in order to build more connected, cross-linked and digitially accessible Internet content it is necessary to assign recognizable, persistent, globally unique, stable identifiers to ... data objects. . The model proposed here for a BUOI aims to make it fully recognizable, universally unique, stable, but always in a well-known context, seldom alone, and with great potential for backup.


  1. Berners-Lee, Tim (2009). Linked Data https://www.w3.org/DesignIssues/LinkedData.html

  2. Clark, J. (2016). PIDvasive:_What's possible when everything has a persistent identifier? PIDapalooza, November 10, 2016. Retrieved Jan 16, 2017. http://dx.doi.org/10.6084/m9.figshare.4233839.v1

  3. Coyle, K. et al.(2014). How Semantic Web differs from traditional data processing. RDF Validation in the Cultural Heritage Community. International Conference on Dublin Core and Metadata Applications, Austin, Oct. 2014. Date accessed: 24 Mar. 2017. http://dcevents.dublincore.org/IntConf/dc-2014/paper/view/311

  4. Doorn, P., Dillo, I. (2017). Assessing the FAIRness of Datasets in Trustworthy Digital Repositories: A Proposal. IDCC Edinburgh, 22 February 2017. http://www.dcc.ac.uk/webfm_send/2481

  5. Duerr, R.E. et al. (2011). (2011). On the utility of identification schemes for digital earth science data: an assessment and recommendations . Earth Science Informatics 4:139. ISSN: 1865-0473 (Print) 1865-0481 (Online) http://dx.doi.org/10.1007/s12145-011-0083-6

  6. Dunning, A., de Smaele, M., Böhmer, J. (2017). Are the FAIR Data Principles fair? Practice Paper. 12th International Digital Curation Conference (IDCC 2017), Edinburgh, Scotland, 20 - 23 February 2017. https://doi.org/10.5281/zenodo.321423

  7. Force11 (2016a). The FAIR Data Principles. https://www.force11.org/group/fairgroup/fairprinciples

  8. Force11 (2016b). Guiding Principles for Findable, Accessible, Interoperable and Re-usable Data Publishing version B1.0. https://www.force11.org/fairprinciples

  9. Data Citation Synthesis Group, Martone M. (ed.)(2014). Joint Declaration of Data Citation Principles San Diego, CA: FORCE11 https://www.force11.org/group/joint-declaration-data-citation-principles-final

  10. Force11 (2016). Guiding Principles for Findable, Accessible, Interoperable and Re-usable Data Publishing version b1.0 San Diego, CA: FORCE11 https://www.force11.org/node/6062/#Annex6-9

  11. Gertler, A., Bullock, J. (2017). Reference Rot: An Emerging Threat to Transparency in Political Science. The Profession. http://dx.doi.org/10.1017/S1049096516002353

  12. Global Names Architecture - GNA (2015). New UUID v5 Generation Tool -- gn_uuid v0.5.0. http://globalnames.org/news/2015/05/31/gn-uuid-0-5-0/

  13. Guo, Xinjiang (2016). Yale Persistent Linking Service PIDapalooza, November 10, 2016. Retrieved Jan 16, 2017. http://dx.doi.org/10.6084/m9.figshare.4235822.v1

  14. Guralnick, R. et al. (2015). Community Next Steps for Making Globally Unique Identifiers Work for Biocollections Data. ZooKeys 494: 133–154. http://dx.doi.org/10.3897/zookeys.494.9352

  15. Hayes, C. (2016). oaDOI: A New Tool for Discovering OA Content. Scholars Cooperative, Wayne State University. http://blogs.wayne.edu/scholarscoop/2016/10/25/oadoi-a-new-tool-for-discovering-oa-content/

  16. Hennessey, J., Xijin Ge, S. (2013). A Cross Disciplinary Study of Link Decay and the Effectiveness of Mitigation Techniques. Proceedings of the Tenth Annual MCBIOS Conference. BMC Bioinformatics, 14(Suppl 14):S5.http://dx.doi.org/10.1186/1471-2105-14-S14-S5

  17. Hornby, N. (1999). About a boy. Indigo, 1999.ISBN: 978-0-575-40229-4.

  18. Klein, M., Van de Sompel, H., Sanderson, R., Shankar, H., Balakireva L., Zhou, K., Tobin, R. (2014). Scholarly Context Not Found: One in Five Articles Suffers from Reference Rot. PLoS ONE 9(12): e115253. http://dx.doi.org/10.1371/journal.pone.0115253

  19. Kunze, J., Russell, M. (2006). Noid - search.cpan.org. http://search.cpan.org/~jak/Noid/noid

  20. Jones, SM., Van de Sompel, H., Shankar, H., Klein, M., Tobin, R., Grover, C. (2016). Scholarly Context Adrift: Three out of Four URI References Lead to Changed Content. PLoSONE 11(12): e0167475. http://dx.doi.org/10.1371/journal.pone.016747

  21. Page, R. (2016). Towards a biodiversity knowledge graph. Research Ideas and Outcomes 2: e8767 (07 Apr 2016). http://dx.doi.org/10.3897/rio.2.e8767

  22. Paskin, N. (1999). Toward Unique Identifiers. Proceedings of the IEEE 87(7):1208 - 1227. http://dx.doi.org/10.1109/5.771073

  23. Patterson, D. et al. (2016). Challenges with using names to link digital biodiversity information. Biodiversity Data Journal 4: e8080 (25 May 2016). http://dx.doi.org/10.3897/BDJ.4.e8080

  24. SESAR - System for Earth Sample Registration (2017). What is the IGSN? http://www.geosamples.org/aboutigsn

  25. Wikipedia (2017a). Link rot. (last modified on 13 March 2017, at 17:46. Retrieved 2017-03-14.) https://en.wikipedia.org/wiki/Link_rot

  26. Wikipedia (2017b). Universally unique identifier. (last modified on 29 January 2017, at 15:28. Retrieved 2017-01-30.) https://en.wikipedia.org/wiki/Universally_unique_identifier

  27. Van de Sompel, H., Klein, M., Jones, S.M. (2016). Persistent URIs Must Be Used To Be Persistent. WWW 2016. arXiv:1602.09102v1 [cs.DL] 29 Feb 2016

  28. Van de Sompel, H. (2016). A Signposting Pattern for PIDs. PIDapalooza, Reykjavik, November 2016. http://dx.doi.org/10.6084/m9.figshare.4249739.v1

  29. Wass, J. (2016). When PIDs aren't there. Tales from Crossref Event Data. PIDapalooza, Reykjavik, November 2016. Retrieved: 11:57, Mar 20, 2017 (GMT). http://dx.doi.org/10.6084/m9.figshare.4220580.v1

  30. Wass, J. (2017). URLs and DOIs: a complicated relationship. CrossRef Blog, 2017 January 31. https://www.crossref.org/blog/urls-and-dois-a-complicated-relationship/

  31. Weitz, C., Weitz, P. (2002) About a boy. http://www.imdb.com/title/tt0276751/. http://core.collectorz.com/movies/about-a-boy-2002. EAN: 3259190282520

  32. Wilkinson, M. D. et al. (2016). The FAIR Guiding Principles for scientific data management and stewardship. Scientific Data 3:160018. http://dx.doi.org/10.1038/sdata.2016.18

  33. Wimalaratne, S. et al. (2015). SPARQL-enabled identifier conversion with Identifiers.org Bioinformatics, 31(11), 2015, 1875–1877. http://dx.doi.org/10.1093/bioinformatics/btv064

  34. Zhou, K. et al. (2015). No More 404s: Predicting Referenced Link Rot in Scholarly Articles for Pro-Active Archiving. In: Proceedings of the 15th ACM/IEEE-CE on Joint Conference on Digital Libraries. JCDL '15, p. 233-236. http://dx.doi.org/10.1145/2756406.2756940