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ERME 7460 Interpretation & Evaluation of Research


Impact factors

What is the Impact Factor?

Originally created by ISI (the Institute for Scientific Information, the Impact Factor is the most widely-recognized method for attempting to gauge a journal's rank/importance.  It is particularly well-known in the Sciences and Social Sciences.

The impact factor is based on two figures:  the number of citations to a given journal over the previous two years (A) and the number of research articles and review articles published by that journal over the same two-year period (B), so: A/B = Impact Factor

Note that there is a discrepancy between the type of content counted for each factor: A = any type of content (including letters, news items, etc.), but B = research or review articles only, making the Impact Factor not a true average.

Where to Find It

Find the Impact Factor for your journal using the various Thomson Reuters products listed below.  Impact factors are also usually listed on individual journal publisher pages.

Use with Care

Informed and careful use of these impact data is essential. Users may be tempted to jump to ill-formed conclusions based on impact factor statistics unless several caveats are considered.
When considering the use of the impact factor (IF), keep these aspects in mind:

  • IF analysis is limited to citations from the journals indexed by the Web of Science/Web of Knowledge.  Currently, the Web of Science indexes only 8621 journals across the full breadth of the sciences, and just 3121 in the social sciences.
  • A high IF/citation rate says nothing about the quality -- or even, validity -- of the references being cited.  Notorious or even retracted articles often attract a lot of attention, hence a high number of citations. The notority related to the first publication on "cold fusion" is one such example.
  • Journals that publish more "review articles" are often found near the top of the rankings.  While not known for publishing new, creative findings, these individual articles tend to be heavily cited.
  • The IF measures the average number of citations to articles in the journal -- given this, a small number of highly-cited articles will skew the figure.
  • It takes several years for new journals to be added to the list of titles indexed by the Web of Science/Web of Knowledge, so these newer titles will be under-represented.
  • It's alleged that journal editors have learned to "game" the system, encouraging authors to cite their works previously published in the same journal.

Impact factors have often been used in advancement and tenure decision-making.  Many recognize that this is a coarse tool for such important decisions, and that a multitude of factors should be taken into account in these deliberations.

Source: Clarivate


Comparing Journals Across Disciplines -- Don't Do It

Using Impact Factors within a given discipline should only be done with great care, as described above.  Using impact factor data to compare journals across disciplines is even more problematic.  Here are some of the reasons:

  • Disciplines where older literature is still referenced, such as Chemistry and Mathematics, offer challenges to the methodolgy since older citations (older than two years) are not used to calculate the impact factor for a given journal.  (Five-year impact factor analysis, which can be calculated using the Journal Citation Index database, helps smooth out this problem only to some degree.)
  • Different disciplines have different practices regarding tendency to cite larger numbers of references.  Higher overall citation rates will bump upward impact factor measurements.
  • Where it's common for large numbers of authors to collaborate on a single paper, such as in Physics, the tendency of authors to cite themselves (and in this case, more authors) will result in increased citation rates.

Finding Impact Factors Using Journal Citation Reports

Access Journal Citation Reports, then select either the Science or Social Sciences Edition and Year.

  1. Search by Subject Category or for a specific journal.  The same journal may appear under several categories, and its impact will vary according to the group.

JCR Main

2. Search by Subject Category and choose Sort by Impact Factor.

JCR Subject

3. Here are the results for a search in the Behavioral Sciences category (shown in two parts due to the width of the display):

JCR Subj I

JCR Subject II

You'll see that additional metrics, such as the Eigenfactor and Article Influence scores, are also captured in this display.

From the Journal Citation Reports Help pages:

Eigenfactor Score

The Eigenfactor Score calculation is based on the number of times articles from the journal published in the past five years have been cited in the JCR year, but it also considers which journals have contributed these citations so that highly cited journals will influence the network more than lesser cited journals.  References from one article in a journal to another article from the same journal are removed, so that Eigenfactor Scores are not influenced by journal self-citation.

Article Influence Score

The Article Influence determines the average influence of a journal's articles over the first five years after publication.  It is calculated by dividing a journal’s Eigenfactor Score by the number of articles in the journal, normalized as a fraction of all articles in all publications.  This measure is roughly analogous to the 5-Year Journal Impact Factor in that it is a ratio of a journal’s citation influence to the size of the journal’s article contribution over a period of five years.

The mean Article Influence Score is 1.00. A score greater than 1.00 indicates that each article in the journal has above-average influence. A score less than 1.00 indicates that each article in the journal has below-average influence.

Resources Discussing the Impact Factor