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Traditionally, the research impact of a researcher is measured by the number of publications he or she has published in peer-reviewed journals, the citation counts his or her papers have received, and the researcher’s H-index. These metrics are usually gathered from citation databases such as Web of Science, Scopus and Google Scholar. Below are a brief introduction of these common research impact indicators used by citation databases.
A. Number of publications
One main indicator of a researcher’s output is the number of publications he/she has published, which is used to reflect his/her research productivity. Publication normally includes peer reviewed and officially published journal articles, conference proceeding papers, books and book chapters. Depending on research disciplines, other forms of publications, e.g. technical reports, working papers, commentaries, textbook might also be accepted as academic publications.
B. Citation counts (including/excluding self-citations)
The number of citations counts refers to the number of times a paper has been cited by other papers, which is used to measure the impact of a publication or author. Self-citations counts typically refer to the number of times an author has cited him/herself in other papers that he/she has authored. Though this can be controversial, high self-citations counts are usually frowned upon as it can be seen as a researcher’s attempt to inflate his/her own citation counts and impact by citing his/her own work frequently when publishing. Some citation databases like Scopus have functions in their databases to differentiate between citation counts that include and exclude such self-citations.
H-index is an author-level metric that attempts to measure both the productivity and impact of a researcher. It is often used as a yardstick to gauge the impact of an individual researcher’s publications. Introduced by Hirsch (2005) to quantify a researcher’s research output, h-index is “defined as the number of papers with citation number ≥h”. 1 For example, a researcher with an h-index of 8 implies that he/she has 8 papers with at least 8 citation counts.
H-index can be used to compare the productivity and citation impact of researchers with comparable years of academic experience and of similar/same field of research. However, it has limited value when used to compare researchers from different subject areas or disciplines as different research fields have different citation and referencing behaviours.
In addition, h-index will not be very useful when evaluating researchers of disciplines or subject areas where the research output is mainly books and other non-article outputs. For example, book and book chapters are typically not covered well in citation databases.