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Manage Your Research and Evaluate Your Sources: Impact Factors and Citation Counts

Covers source evaluation systems, metrics, impact factors, plagiarism, and arranging your materials

What are metrics?

How does the scientific community measure how "good" or "great" a journal or an author is? How do you determine the "impact" of an author's work? Should it be purely based on the number of times the article is cited? If not, how can we also note (or measure) the "quality" of the research?

Several methods to calculate the impact or "good-ness" of an article, journal, or author have been developed answer these questions. These calculations and statistical methods are called metrics. Be aware metrics are highly debated. The most popular metrics include: number of citations (journal or author), journal impact factor, and author h-index. There are hundreds of other metrics available, some better defined than others.

Journal Impact Factors

One metric used is impact factor. According to Journal Citation Reports (JCR), the impact factor is a ratio focusing on original research. 

Impact factor = # of citations to all items published in the past two years (divided by)
                        # of articles and reviews published over those past two years.

For example, if a journal has an impact factor of 2.5, this means in the indexed year each article published was cited on average 2.5 times in the previous two years.

Impact factor is used for journals only.

Author Impact / Citations

The most common metric to track authors is ask how often they are cited. However, to find that information, a database must be able to read the references (cited articles) in every article.This is only possible if the database has access to the full text of all articles an author publishes. There are nearly 35,000 journals that have at least one article in PubMed; Web of Science searched 10,000 "high impact" journals of the over 24,000 in their database; the specialized database, PsycINFO includes 2,395 journals.

Author H-index

One calculation of an author's impact is the h-index. The h-index is somewhat complex calculation taking into account the total number of articles published by an author and the number of citations per article.

An author's h-index, or h, is equal to the number of total articles published--as long as every article has at least h number of citations.

h-index is used for authors only.

Cited Articles = Confusing Statistics

It's easy to say "how many times has this article been cited?" However, there is no simple answer. This number depends entirely on the database used and how the article references are searched by that database. Take, for example, for the article:

Age differences activity during emotion processing: reflections of age-related decline or increased emotion regulation? by Kaoru Nashiro, Michiko Sakaki, and Mara Mather.

On May 1, 2017:

  • In PubMed, this article was cited 31 times in PMC (an archival database, formerly PubMed Central containing 4.3 million full-text articles)
  • Google Scholar reports: Cited by 90 (with 22 available versions) - the date is incorrect (2011), but the link is to the correct article (Google Scholar searches openly web accessible resources and collects duplicates as versions, includes non-English citations)
  • Scopus says Cited by 49 documents (a database of over over 21,500 peer-reviewed journals plus conference papers, books and patents)
  • Web of Science reports the article was 37 times cited (a database searching 10,000 high-impact journals and additional international conference proceedings)
  • ResearchGate reports 70 citations (a database of author uploaded articles)

Each database or search engine has strengths and limitations. Some (e.g. Google Scholar and ResearchGate) include duplicate listings in their citation numbers. To identify all of the articles, dissertations and books that cite an article, it may be necessary to search multiple databases and keep your own records.