The impact of Grey Literature in the web environment: a citation analysis using Google Scholar

The use of Grey Literature (GL) has hitherto been studied on the basis of whether and to what extent GL documents had been cited in peer-reviewed conventional literature applying citation analysis techniques based on the multidisciplinary citation indexes produced by Thomson ISI, the Web of Science (WoS). More recently, other tracking citation systems have been developed, such as Scopus, Google Scholar, Citeseer, and CrossRef. The novelty of some of these new systems is that they consider not only cited articles of peer-reviewed conventional journals, but also include the number of citations received by GL documents. This opens up new research perspectives in citation analysis, which permit a better measurement of GL use as well as of its impact on scholarly communication. The paper presents the results of a citation analysis with the aim of evaluating the use of GL through a comparison of the number of citations received by both GL and conventional documents in a pre-defined scientific topic. Moreover, the paper aims to apply some bibliometric indicators, which allow us to identify different citation behaviours and verify whether they are related to the type of documents cited, to the disciplinary field and/or to the availability of the full text. The analysis is carried out retrieving documents dealing with the topic population ageing reported in Google Scholar. In the first step of the study, 100 most cited documents are analysed selecting them from the approximately 2000 documents retrievable in Google Scholar. In particular, the analysis focuses on the identification of: a) GL and conventional literature, b) type of cited documents, c) authorship, d) year of publication. In the second step of the study, a significant sub-set of the above cited documents are further analysed taking their citing documents into account. Bibliometric indicators, such as the immediacy index, the average citations per year and the cited half-life are used in order to identify differences in citing behaviours in both GL and conventional literature. Moreover, the topic population ageing has the advantage of being studied under various perspectives, such as demographic, socio-economical or medical aspects and this allows us to verify whether there are differences in the type of the citing documents related to the disciplinary field. Another overall result of the study is the evaluation of Google Scholar, in terms of the consistency of retrievable information as well as of the additional tools, which may provide value added information to perform citation analysis.