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Impact Factors

What's a Journal Impact Factor?
Journal Impact Factors can be thought of as the average number of citations a journal's articles from two to three years ago received last year from all journals in the citation index. Journal Impact Factors are computed by taking the number of citations received in the reporting year to articles published in the preceding two years and dividing it by the number of content articles the journal published during those two years.[1]

What's a good Journal Impact Factor?
Journal Impact Factors vary widely by subject.[2]  A "good" number in one subject area may look like a "bad" number in another subject. To know whether a journal's Impact Factor is high for its subject, compare it to Journal Impact Factors of other journals in that same subject. 

What does it mean if a journal doesn't have a Journal Impact Factor?
Thomson-Reuters goes through a selection process to decide which journals are included in the database.[3] A journal without a Journal Impact Factor could...

  • be in a subject area that JCR doesn't include for Journal Impact Factors,

  • be too new to have the years of citations needed to compute a Journal Impact Factor,

  • be in a language other than English, or

  • otherwise not meet Thomson-Reuters criteria for inclusion. 

Why doesn't the Impact Factor on my CV match the Journal Impact Factor in Journal Citation Report? I swear I copied the number correctly when I looked it up last year.
Journal Impact Factors change over time because they are based on citations from a recent year to articles in the previous two years. On average, journal Impact Factors have tended to go up over time.[2] If you are regularly reporting Journal Impact Factors, be consistent about whether you are reporting current Journal Impact Factors or Journal Impact Factors from the time of publication. 

What are the problems with the Journal Impact Factor?
Criticisms of Thomson-Reuters' Impact Factor include whether citations are the right thing to measure, whether the Journal Impact Factor measures citations in the right way, whether the Journal Impact Factor is interpreted correctly when it is used.

  • Misuse to assess individuals rather than journals: Journal Impact Factors provide information about journals, but they sometimes are misused to assess things other than journals.[4, 5]
  • Skewed distribution: In general, a small number of articles contribute disproportionately to the total number of citations a journal receives. Despite being an "average," journal's Impact Factor is not representative of the number of citations a typical article receives.[6]
  • Discipline variations: Thomson-Reuters doesn't cover all subject areas equally well, and different fields have different customs for how many citations to include in a paper, Journal Impact Factors vary by discipline.[2, 6]  It should not be used to create a single ranked list of journals across disciplines; nevertheless, it sometimes is used this way.
  • Gaming the numbers: To boost the Impact Factor of their journals, some editors ask authors to cite other papers from the same journal or form cartels with other editors to ask authors to cite articles from other journals in the cartel. When discovered, Thomson-Reuters blackballs journals for this unethical behavior.[7]

 What are the other numbers like the h-indexthe five-year Impact Factor, the Eigenfactor, SJR, etc.?
For a long time, Journal Impact Factors were one of the few quantitative ways to assess publications. Recently, as tools for tracking citations became available, alternatives to the Journal Impact Factor emerged. Some of the numbers, like Journal Impact Factors, use citation counts to rank journals, but they apply different methods that the creators believe reduce the problems with Journal Impact Factors. Other numbers, such as the h-index, were devised to assess individual researchers using counts of citations to their works.  

 As of 2013, the situation is unsettled. The alternatives to the Journal Impact Factor may not necessarily avoid all of its problems without creating problems of their own.[4] Aside from the h-index, most of the alternatives to the Journal Impact Factor have not gained wide recognition, so they have not received the same amount of scrutiny as the Journal Impact Factor. On the other hand, some journal editors and scholarly societies have issued a public declaration against the misuse of the Journal Impact Factor.[8] Discontent with way the Journal Impact Factor is being used may open the door for alternatives to take its place.