1. Introduction

Patents, as the main exponent of technological development, contain very valuable information that is used by researchers and analysts to know the state of a technology, its evolutionary trajectory, the relationship with other technologies or the relations between agents involved. Furthermore, patents also provide very interesting information such as the influence of scientific research on technology, this is possible through the analysis of the citations from scientific publications contained in these documents.

In this sense, numerous studies have focused on measuring the impact of these scientific references on patents, considering them a value indicator. Pioneering authors in this field ^{[1][2][3][4][5]} had already used bibliometric procedures to quantify these data and explain the transmission of knowledge from science to industry. 

But the importance of these citations also lies in aspects related to the evaluation of scientific production, since according to Plaza ^[6] from the analysis of these citations information is obtained about the authors/researchers, scientific institutions, journals cited in the patents, among others. Moreover, the fact that they have been cited in these documents would give them added value on the influence of that publication in the technological field. In this sense, the format and standardisation of these references becomes important for the establishment of metrics that allow their quantification and analysis, in order to assess this "technological" factor.

The characteristics of these citations in the patents can be known in the initial phase of the process of granting the patents where they are presented after the search of the Prior Art.

2. Search reports “Prior Art”

In the process of granting the patents, the examiners responsible for evaluating them prepare a report in which they provide previous references from scientific and technological literature, the so-called "State of the art" or “Prior Art” in order to justify the novelty and usefulness of the invention. At this point, a distinction is made between two types of reference, on the one hand, quotations from previous patents (Patent Literature) and, on the other, references to other types of documents such as scientific articles, monographs, technical standards, among others, the so-called Non-Patent Literature (NPL).

With regard to the authorship of citations, they may be mentioned by the applicant/inventor in the description of the patent, or by the examiner evaluating the process, which need not coincide with those provided by the applicant, as he may omit them or add more references of interest to the process ^[7].

3. Taxonomy.

Within the NPL document set, scientific articles are the most commonly cited by patents. For this reason, the author Callaert ^[7] takes them as the basis for his taxonomy, which is described below:

Table 1. Taxonomy. Reference types ^[7]

Other authors ^[8] adapt this taxonomy and distinguish between references to Science “at large” (References to scientific publications, Conference Proceedings and Books ) y Technology “at large” (Industry / Company related documents and Patent related documents).

4. Format and standardisation

The format of the citations is regulated by international standards, commonly adopted as Standard ST.14 "Recommendation for the inclusion of references cited in patent documents" of the World Intellectual Property Organization (WIPO). In reference to the NPL citations, in the last revision of 2016, it establishes its bibliographic format according to the International Standard ISO 690:2010 "Information and documentation - Guidelines for bibliographic references and citations to information resources".

This standard also includes the categorisation of citations within the patent document according to their relevance, so that they are assigned different letters if the citations show characteristics of the invention or show technological backgrounds.

Although an international regulation exists, it is not always applied with the same rigour, as this will depend, to a large extent, on the degree of exigency of each organism. In this sense, some authors ^[9] have detected evident problems of lack of standardisation regarding the purging of authorities (names of authors) in NPL references.

5. Cooperation projects. Common citation portals.

Cooperation on patent subjects has been developed over the years by the world's leading patent offices, providing open access to both their own data collections and shared collections of other offices.

With regard to citations, there is the Five IP Office cooperation project, composed of the offices in Europe, EEUU, Japan, Korea and China, whose aim is to improve patent examination processes, with services and applications such as the search tool Common Citation Document (CCD) or the Global Dossier initiative, which provides access to documents associated with the patent granting process, including lists of bibliographic references submitted by the applicant and examiner, as well as the terms and classifications used in the Prior Art search. Access to Global Dossier is through the USPTO or Espacenet (European Patent Office) portals.

6. NPL impact.

As mentioned above, through the analysis of NPL citations, indicators are obtained to measure the impact of these citations from two fields: scientific and technological.

With regard to the scientific impact, the measurement of NPL citations in patents indicates the degree of scientific intensity in these documents, as well as other aspects such as the inference of scientific research in certain technological sectors or the level of industrial application of each country's own research.

But these NPL citations also provide information on the technological impact of the scientific production cited in the patents, on questions related to the productivity of the authors, impact of the journals or maps of influence of research institutions.

Figure 1. Non-Patent Literature (Gema Velayos-Ortega, 2020).

In this sense, there are numerous works that have studied NPL citations on a specific aspect or a specific subject, as is the case of a work in which analyse the citations of scientific publications in the patents on coronavirus, obtaining, among other results, a ranking of the most cited journals in these patents, on which they make a comparison with their positioning in JCR of the Web of Science ^[10].

From this perspective, the measurement of NPL citations with indicators different from the traditional bibliometrics could be considered as an assessment element to be taken into account in the evaluation systems of science and technology.