Despite some limitations, the number of citations received is one of the generally accepted indicators in measuring the impact of an article. A high number of citations is associated with a greater impact.
It is often useful for researchers to emphasize the number of citations received for their articles in order to demonstrate the impact they have had on their given fields.
Citation practices vary from one field to the other: on average, an article on a biology topic will be cited more often than an article on a mathematics topic. Therefore, one should not compare the impact of two articles from two different fields solely on the absolute number of citations received.
Access to Citation Databases
Several databases provide the number of citations received by an article:
Scopus: Launched in the early 2000s, it is a direct competitor of Web of Science. Polytechnique Library does not subscribe to this resource.
IEEE Xplore: Does not have tools for analyzing citations, but provides the citations received by a specific article. Especially useful for IEEE articles and conferences that are not always included in Web of Science.
Google Scholar: Free resource; however the number of citations is often overestimated (duplicate entries, non-academic sources, etc.).
Important: The coverage of each database is different, therefore the number of citations varies from one database to another.
The h-index is an index developed by J.E. Hirsch in 2005. It is a unique indicator that measures both the productivity (number of articles published) and the impact (number of citations received) of a researcher or scholar. The h-index indicates the number of publicationsn that were cited at least n times.
Just like all other indicators, the h-index has certain limitations:
It varies greatly from one discipline to the other, so one should never use it to compare researchers from different disciplines;
It varies significantly with the age and career length of a researcher;
It does not take into account the author order or the number of authors of an article;
It ignores excessive citations (an author who has published only 1 article that has received 300 citations has nevertheless an h-index of 1).
The Library has developed a tutorial presenting the steps to find an h-index using the Web of Science database.
Google Scholar can also be used to find an h-index. However, a panel of experts mandated by the Council of Canadian Academies affirmed that, "Google Scholar should not be used as a data source for rigorous bibliometric assessment." (Informing Research Choices: Indicators and Judgment , 2012, p. 60).
To manually find an h-index, place the articles in descending order of citations and find where the rank of the article is higher than the number of citations. The h-index is the rank immediately before. In the example below, the h-index is 7.
Figure 1: h-Index calculated from a descending number of citations. (2008) Public domain.
Fractional counting can be used to count an author’s publications. According to this method, each article counts as 1 ÷ (number of authors). Fractional counting can also be used for citations.
Furthermore, it can be useful to assess the number or percentage of articles an author has written in collaboration. This collaboration can be interinstitutional, international or trans-sectoral. The “Analyze data” tool in Web of Science is one of the ways to assess collaboration.
The Relative Citation Impact (IRC) compares the citations received with the average global citations for a given field. This indicator can be used for citations received for a particular article, even though it is often used to compare the average of all the citations received for an author, institution, or even a country with the world average. This way of quantifying citations is more appropriate for comparing researchers from different fields.
Several researchers have developed indicators to complement the information from the h-index. The most recognized are the g-index and e-index, which give information on citations that go beyond the h-index. It has also been suggested that the h-index should cover 5 years to make it easier to compare researchers who are at different stages in their careers.
Also, altmetrics are increasingly used to measure impact.