Last month we introduced the concept of impact factor, a measure of a journal's importance (or, impact) based on the frequency with which the "average article" in the journal is cited over a particular time period. Now we will look at how impact factors are calculated.
Impact factors are compiled by ISI and published in Journal Citation Reports (JCR), a resource for evaluating and ranking journals. Citations to journals listed in JCR are compiled annually. Each unique article-to-article link is counted as a citation, regardless of the type of article cited or the publication date of the article cited. For example, an article may have forty references, but five of them contain previously cited references. Only the thirty-five unique citations are distributed among the cited journals.
The formula for calculating impact factor includes the following:
A = total cites in [JCR publication year]
B = [JCR publication year] cites to articles published in previous two years (a subset of A)
C = number of articles published in previous two years
D = B/C = [JCR publication year] impact factor
FOR EXAMPLE:
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Impact Factor Calculation
Journal: Nature
Impact Factor: 25.814
Cites in 2000 to articles published in:
1998 = 30512
1999 = 21013
1998 + 1999 = 51525
Number of articles published in:
1998 = 980
1999 = 1016
1998 + 1999 = 1996
Calculation:
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Cites of recent articles
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=
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51525
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|
Number of recent articles
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=
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1996
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=
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25.814
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|
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Impact Factor: The average number of times recent articles in a specific format were cited in the JCR cover year. (Recent articles are defined as those published in the two years preceding the JCR cover year.)
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ISI also calculates the immediacy index, another factor in ranking journals.
FOR EXAMPLE:
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Immediacy Index Calculation
Journal: Nature
Impact Factor: 4.389
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Cites in 2000 to articles published in 2000
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=
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5771
|
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Number of articles published in 2000
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=
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1315
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Calculation:
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Cites to current articles
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=
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5771
|
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Number of current articles
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=
|
1315
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=
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4.389
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|
|
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Immediacy Index: The average number of times current articles in a specific journal were cited during the year they were published. (Current articles are defined as those published in the JCR cover year.) An article published early in the year has a better change of being cited that year than one published later in the year.
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Source for calculations: 2000 Journal Citation Reports (ISI Thomson Scientific)
Next month: Applications
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