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Journal Metrics: ISI's JIF

This guide describes what the journal impact factor (JIF or IF) is, criticisms of IF, how to use IF responsibly, and other journal IFs being developed by other organizations.

Journal Impact Factor

Journal Impact Factor is frequently used as a proxy or ranking system for the importance of a journal to its field.  Journal Citation Reports, Eigenfactor, SCImago Journal & Country Rank and a few web based resources such as F1000 and Google Scholar are tools for finding the impact of a journal or a group of journals.


FYI:  a few definitions of what you will see in the column headings when you look for a journal in JCR (Most of these definitions are taken directly from the HELP files of JCR.  For more definitions, go to JCR Web from the LHL Databases page and click on the Help button):

5-Year Journal Impact Factor:  the average number of times articles from the journal published in the past 5 years have been cited in the JCR year.  It is calculated by dividing the number of citations in the JCR year by the total number of articles published in the 5 previous years.

The Immediacy Index is the average number of times an article is cited in the year it is published.

  • The journal Immediacy Index indicates how quickly articles in a journal are cited.

  • The aggregate Immediacy Index indicates how quickly articles in a subject category are cited.

The Immediacy Index is calculated by dividing the number of citations to articles published in a given year by the number of articles published in that year.

Because it is a per-article average, the Immediacy Index tends to discount the advantage of large journals over small ones.   However, frequently issued journals may have an advantage because an article published early in the year has a better chance of being cited than one published later in the year.  Many publications that publish infrequently or late in the year have low Immediacy Indexes.

For comparing journals specializing in cutting-edge research, the immediacy index can provide a useful perspective.

The aggregate Impact Factor for a subject category is calculated the same way as the Impact Factor for a journal, but it takes into account the number of citations to all journals in the category and the number of articles from all journals in the category.   An aggregate Impact Factor of 1.0 means that that, on average, the articles in the subject category published one or two years ago have been cited one time.  The median Impact Factor is the median value of all journal Impact Factors in the subject category.

The Impact Factor mitigates the importance of absolute citation frequencies.  It tends to discount the advantage of large journals over small journals because large journals produce a larger body of citable literature.  For the same reason, it tends to discount the advantage of frequently issued journals over less frequently issued ones and of older journals over newer ones.  Because the journal impact factor offsets the advantages of size and age, it is a valuable tool for journal evaluation.

 Journal Cited Half-Life

The median age of the articles that were cited in the JCR year.  Half of a journal's cited articles were published more recently than the cited half-life.  For example, in JCR 2001 the journal Crystal Research and Technology has a cited half-life of 7.0. That means that articles published in Crystal Research and Technology between 1995-2001 (inclusive) account for 50% of all citations to articles from that journal in 2001.

Only journals cited 100 or more times in the JCR year have a cited half-life.

A higher or lower cited half-life does not imply any particular value for a journal.  For instance, a primary research journal might have a longer cited half-life than a journal that provides rapid communication of current information.  Cited half-life figures may be useful to assist in collection management and archiving decisions.  Dramatic changes in cited half-life over time may indicate a change in a journal's format.  Studying the half-life data of the journals in a comparative study may indicate differences in format and publication history.

 Journal Citing Half-Life

The citing half-life is the median age of articles cited by the journal in the JCR year.  For example, in JCR 2003, the journal Food Biotechnology has a citing half-life of 9.0.  That means that 50% of all articles cited by articles in Food Biotechnology in 2003 were published between 1995 and 2003 (inclusive).

Only journals that publish 100 or more cited references have a citing half-life.  Cited-only journals do not have a citing half-life.



Journal Citation Reports (JCR)

The JCR provides quantitative tools for ranking, evaluating, categorizing and comparing journals.  The IF 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.

JIF is a measure of the influence that a particular  journal has in its field.  Researchers often want to publish in journals that have a higher impact factor so that their individual articles have a higher chance of being seen and referenced.  The journal's impact factor does not necessarily indicate anything about the particular authors published in it, except that it may be more prestigious to publish in a higher impact factor journal.

JIF can be used to (1) judge a publication's quality or prestige; (2) assess academic productivity; (3) authors choosing where to publish; (4) evaluate an author or journal editor; (5) decisions for tenure & promotion; and (6) libraries use JIF to make collection development decisions.

The impact factor of a journal in a particular year is the number 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 2010 impact factor of 5.391, which means that on average each of its 2008 and 2009 articles was cited 5.391 times in 2010.  The information below for Pediatrics is copied from JCR 2010.

Journal Impact Factor can be used to identify:

  • Most frequently cited journals in a field.
  • Highest impact journals in a field.
  • Leading journals in a field.
  • Related journals in a field.
  • Citation characteristics for a subject category.

Cites in 2010 to items published in: 2009 = 3404    Number of items published in: 2009 = 752 
  2008 = 4543      2008 = 722 
  Sum: 7947      Sum: 1474 
Calculation: Cites to recent items  7947 = 5.391 
  Number of recent items  1474     

 Note that Pediatrics is at the top of the list in the field of pediatrics.  Which is why you would not compare it with a general science journal (or multidisciplinary sciences journal, to use the JCR subject category) like Nature which has an impact factor of 36.104.  Remember apples to apples, oranges to oranges is the watchword when using any kind of bibliometrics to evaluate an individual, an article or a journal.


The following limitations and biases of the Journal Impact Factor were compiled by the University of Oulu (Finland) and appear in their Toolbox of Research.  (Their Toolbox is an outstanding resource and is available to all.)

  • All citations are weighted equally regardless of the prestige of the citing journal.
  • Differences in citation patterns among disciplines are not considered, which makes it impossible to compare journals across disciplines.
  • The context of citations is not considered.  Articles are cited for various reasons, and often a citation does not reflect the scientific merit of the cited work.  Negational citing is especially related to disciplines with a critical discourse.  Thus a high citation rate of an article may not always be associated with high quality.
  • Only articles that are cited within two years after publication contribute to the impact factor although many important papers achieve their maximal scientific impact outside of this time-frame.
  • Citations made in journals not recorded in the Web of Science database do not contribute to impact factor calculations.
  • Citations are gathered from journal articles only, and not from books, book chapters, or conference proceedings.
  • The denominator of the Impact Factor calculation is made up by citable articles.  The non-citable items, including letters, news stories, abstracts, book reviews, and editorials, are not included in the denominator of the impact factor equation but may be included in the numerator if they are, although infrequently, cited.
  • The sample of journals included in JCR is limited, and represents only a small fraction of all journals.  In addition, the journal selection process is somewhat ambiguous.  The content of the JCR is not static, but the journal evaluation and selection process is an ongoing process.
  • JCR has a preference for the English language, and is dominated by North American publications.  Journals that publish articles in languages other than English will likely receive fewer citations because a large portion of the scientific community cannot read them.
  • Self-citations are included in total cites of a journal.  A high self-cited rate indicates a journal's low visibility, and a high self-citing rate is an indicator of the isolation of the field covered by the journal.  Thomson Reuters takes into consideration the self-citation rates of journals it covers, and an unusually high rate of self-citations can lead to exclusion of the journal.  The total citation counts also include citations of authors to their own work but several citations to the same article in one article are counted only once.
  • Minor errors in the preparation of article reference list information (basic typographical mistakes, inconsistent spelling of surnames and use of initials, and so on) may also inadvertently bias citation identification and recording.  It has been suggested that this type of mistake occurs in up to 7% of citations.
  • Comparisons of two journals in different disciplines using Impact Factor are not possible.  The Impact Factor varies considerably among disciplines due to different citation and referencing tendencies of each discipline.  In addition, if in some disciplines many citations occur outside the two-year window, Impact Factors for journals will be low.
  • The Impact Factor of a journal cannot be considered to represent the citation rate of an individual article and does not permit assessment of the quality of an individual article or author.  In fact, there is virtually no correlation between the frequency of citation of an individual article and the Impact Factor of the publishing journal.  The misuse of Impact Factor has, in recent years, widened to include evaluation of the quality of individual papers and even individual authors and research groups.
  • Many authors may be tempted, or feel pressured, to select the highest Impact Factor-rated journals likely to accept their article for publication while rejecting journals whose target audience may in fact be more suitable and receptive to the publication itself.
  • Impact Factor can distort publication polices of journals if they focus on improving their Impact Factors, for example, by publishing more reviews.  An item is classified as a review by Thomson Reuters if it meets any of the following criteria:  it cites more than 100 references, it appears in a review publication or a review section of a journal, the word review or overview appears in its title, or the abstract states that it is a review or survey.

Using JCR

How to find the Journal Impact Factor by Subject Grouping:

1.  Go to JCR

2.  Select a JCR edition year from the dropdown list

3.  Make sure that the radio button next to View a group of journals by Subject Category is selected

4.  Click submit button

5.  Select one or more subject categories (hold down control <Ctrl> key while clicking subjects) (HINT: you can type the first letter of the subject category and you will be sent to the first subject category that begins with that letter.  ALSO, make sure that the subject categories you select are really closely related to each other or the results will be meaningless.  Selecting infectious diseases and medicine, general & internal will not benefit your analysis.)

6.  Select Journal or Category data sorts:

      a.  A journal data sort will list the  journals in the category or categories selected.  If more than one category is selected, the category from which the journal title came is not listed.

      b.  A category data sort will provide you with aggregate information about the categories for comparison purposes.

7.  Click the Submit button.

Using JCR

How to find the Journal Impact Factor for an Individual Journal Title:

1.  Go to JCR

2.  Select a JCR edition year from the dropdown list

3.  Click the radio button to the left of "Search for a specific journal"

4.  Click the Submit button

5.  Enter journal information by full journal title, abbreviated journal title, title word or ISSN

(HINT:  if you are having trouble entering the journal's full name, click on the link 'view list of full journal titles'.  All the titles are in one long list.  Once there, use <Ctrl> + f (the Find feature) to search for your journal title word.  Especially useful for a title like AMERICAN JOURNAL OF PHYSIOLOGY-LUNG CELLULAR AND MOLECULAR PHYSIOLOGY and for journals that begin with an acronym like JAMA.)

6.  Click Search button.

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