HealthLinks | Health Sciences Libraries | How-To

Impact Factors

An Impact Factor is one measure of the relative importance of a journal, individual article or scientist to science and social science literature and research. Each index or database used to create an impact factor uses a different methodology and produces slightly different results, revealing the importance of using several sources to judge the true impact of a journal's or scientist’s work. Included on this page is information on Journal Impact Factor and Author Impact Factor.

Informed and careful use of these impact data is essential, and should be based on a thorough understanding of the methodology used to generate impact factors. There are controversial aspects of using impact factors:

    • It is not clear whether the number of times a paper is cited measures its actual quality.
    • Some databases that calculate impact factors fail to incorporate publications including textbooks, handbooks and reference books.
    • Certain disciplines have low numbers of journals and usage. Therefore, one should only compare journals or researchers within the same discipline.
    • Review articles normally are cited more often and therefore can skew results.
    • Self-citing may also skew results.
    • Some resources used to calculate impact factors have inadequate international coverage.
    • Editorial policies can artificially inflate an impact factor.

Journal Impact Factor

Journal Impact Factor is frequently used as a proxy for the importance of a journal to its field. Journal Citation Reports and the Eigenfactor are tools for finding the impact of a journal or groups of journals.

Journal Citation Reports (JCR) UW IP

The JCR provides quantitative tools for ranking, evaluating, categorizing, and comparing journals. The impact factor is one of these; it is a measure of the frequency with which the "average article" in a journal has been cited in a particular year or period. [Garfield, 2005] Journal impact factor applies only to a journal or groups of journals, but not to individual articles or individual researchers.

The impact factor of a journal in a particular year is the number of citations received in the current year to articles published in the two preceding years divided by the number of articles published in the same two years. For example, Pediatrics has a 2006 impact factor of 5.012, which means that on average each of its 2004 and 2005 articles was cited 5.012 times in 2006.

How to find the Journal Impact Factor by individual journal title or by subject groupings:

Subject:

    1. Go to JCR.
    2. Select a JCR edition year from dropdown list. (leave view group of journals by subject category).
    3. Click Submit button.
    4. Select one or more subject categories (hold down control <ctrl> key while clicking subjects).
    5. Select Journal or Category data sorts. Under View Journal Data, select Impact Factor.
    6. Click Submit button.

Individual Journal Title:

    1. Go to JCR.
    2. Select a JCR edition year from dropdown list.
    3. Click radio button to the left of ‘Search for a specific journal’.
    4. Click Submit button.
    5. Enter journal information by complete title, ISSN, abbreviated journal title, or title word.
    6. Click Search button.

In results table click individual journal title for complete information (e.g., explanations of impact factor, immediacy index, citing and cited half-life)

For more details see:

Journal Citation Reports tutorial (from Thomson ISI Web of Knowledge)
www.scientific.thomson.com/tutorials/jcr4

Journal Impact Factors section in Citation Analysis with the WoS- Part II (from the Cushing/Whitney Medical Library, Yale University)
www.med.yale.edu/library/education/guides/citation

Eigenfactor

Eigenfactor ranks and maps scientific knowledge:

    • Ranks journals similar to Google ranking of websites. It uses the structure of the entire network (instead of purely local citation information) to evaluate the importance of each journal.
    • Measures journal price as well as citation influence. The Cost-Effectiveness Search orders journals by a measure of the value of the dollar they provide.
    • Ranks scholarly journals as well as newspapers, theses, popular magazines, etc.
    • Adjusts for citation differences across disciplines, allowing for better comparison across research areas.
    • Calculations are based on the citations received over a 5-year period vs. 2 years in JCR.
    • Available free of charge on the web. [eigenfactor.org, 2007]

How to find the eigenfactor:
    1. Go to eigenfactor.org
    2. Search for a single journal name or choose a subject category.
    3. Select a year.
    4. Click Search.

Note: Cost effective (searches by category) and advanced search (searches by Thomson JCR subject categories, publisher, and other fields) are also available.

Author Impact Factor

The Author impact factor calculates the scientific value of a given researcher or author. You can try the h-index or compile cited references by using Web of Science or Google Scholar.

H-Index

The h-index quantifies the actual scientific productivity and the apparent impact of the scientist. The h-index is based on the author’s most cited papers and the number of citations they have received from other articles.

"A scientist has index h if h of his/her Np papers have at least h citations each, and the other (Np − h) papers have no more than h citations each." [For details in calculation, see Hirsch, 2005] An h-index of 16 means, for example, that a researcher has published 16 papers that each had at least 16 citations. Therefore, the h-index reflects both the number of articles as well as the number of citations per article.

How to find the H-index of an individual author in Web of Science:

    1. Go to Web of Science from the Reference page or Databases list.
    2. Enter author’s name and be sure the pull-down box indicates Author.
    3. Click Search.
    4. Refine Results by subject areas or other criteria if desired.
    5. Click 'Create Citation Report' link (right side of window, just above results list).
    6. The Citation Report lists the h-index near the top right of the page (Note: ? link for more information).

Cited References

Cited references make it possible to find other documents that are related by topic or subject to the original document. Cited references (references that cite an individual article) may be used to measure the usage and impact of a cited work. Note that cited references can be influenced by author self-citing or publishing in an open access journal.

Citation analysis, which involves counting how many times a paper or researcher is cited, assumes that influential scientists and important works are cited more often than others.

Web of Science (WoS) Cited References

Cited Reference Search is one of the features in the Web of Science database. The number in the Citing Articles column in WoS indicates the number of times the reference has been cited in all years of Web of Science, regardless of how many years you are searching. Note that Citing Article references may not include all the known citations of the paper, just those in journals covered by WoS.

How To Perform a WoS Cited Reference Search

    1. Enter the name of the primary Cited Author and the Cited Year or a limited range of years of a Cited Work, and then click Search. For more details, see images.isiknowledge.com/help/WOS/hcr_citedau.html
    2. If you retrieve too many hits, return to the form and add the abbreviated title of a Cited Work.
    3. After you click Search, you will see references from the citation index that contain the cited author/cited work data you entered. You can note how many times the article or work has been cited in the Citing Articles column.
    4. Select references by checking the box to the left of each reference you want.
    5. To retrieve these citing articles, click Finish Search. You have now retrieved the records of articles that cite the author/reference you selected.
    6. Clicking on Analyze Results allows you to view rankings of the authors, journals, etc. for your set of results.

For further help:

Cited Reference Searching in the Web of Science (from Thomson ISI Web of Knowledge)
www.brainshark.com/brainshark/vu/view.asp?pi=144957

Citation Analysis with the Web of Science,PartII: Cited Ref Search, Author Finder and Impact Factor (from the Cushing/Whitney Medical Library, Yale University) www.med.yale.edu/library/education/guides/screencasts/ca/wos_2/

Google Scholar (Beta) Cited References

Google Scholar (GS) covers peer-reviewed papers, theses, books, abstracts, and other scholarly literature from all broad areas of research and from a wide variety of academic publishers and professional societies, as well as scholarly articles available across the web. Each Google Scholar search result contains bibliographic information, such as the title, author names, and source of publication. At the end of the search result is a “Cited by” link, which will display a list of articles and documents that have cited the document originally retrieved in the search. Note that this only includes resources indexed by Google Scholar.

There have been some criticisms of Google Scholar Cited References, such as:

    • GS includes some non-scholarly citations.
    • It is not clear exactly which scholarly resources are included in GS.
    • GS does not perform well for older publications.
    • GS is a beta product and may change at any time.
    • GS is not updated as often as WoS.

How to find Cited References in Google Scholar:
    1. Go to Google Scholar: scholar.google.com
    2. Enter search terms, such as an individual author or a particular article citation.
    3. Look for the “Cited by” link at the bottom of the citation.
    4. Click on the “Cited by” link to retrieve citations to the original resource.

References

Bergstrom C. Eigenfactor: Measuring the value and prestige of scholarly journals. C&RL News 2007 May; 68(5).
www.ala.org/ala/acrl/acrlpubs/crlnews/backissues2007/may07/eigenfactor.cfm

Eigenfactor.org. Why eigenFACTOR? 2007. eigenfactor.org/whyeigenfactor.htm

Garfield E. The agony and the ecstasy: the history and the meaning of the journal impact factor. International Congress on Peer Review and Biomedical Publication, Chicago, Sept 16, 2005.
garfield.library.upenn.edu/papers/jifchicago2005.pdf
scientific.thomson.com/news/newsletter/2005-11/8298245/

Harzing A-W. Google Scholar - a new data source for citation analysis. 5 Feb 2008.
www.harzing.com/resources.htm#/pop_gs.htm

Hirsch JE. (2005). "An index to quantify an individual's scientific research output," Proceedings of the National Academy of Sciences, 2005 Nov 15; 102(46):16569-16572.
arxiv.org/PS_cache/physics/pdf/0508/0508025v5.pdf

Meho LI. The rise and rise of citation analysis. Physics World, 2007; 20(1), 32-36.
www.slis.indiana.edu/faculty/meho/PhysicsWorld.pdf

Mt. Sinai Gustave L. and Janet W. Levy Library. Publications Metric for 2006. Includes graphic explanations for calculating impact factors.
www.mssm.edu/library/reference/impactfactors.shtml

Rossner M, Van Epps H, Hill E. Show me the data. The Journal of Cell Biology, 2007; 179(6):1091-1092.
www.jcb.org/cgi/content/full/179/6/1091

Creative Commons License
This work is licensed under a Creative Commons License.

Updated 9Apr2008 (js, lsa)