ISI Impact Factor
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. The annual JCR impact factor is a ratio between citations and recent citable items published. Thus, the impact factor of a journal is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years (see Figure 1).
| Figure 1: Calculation for journal impact factor. |
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| A= total cites in 1992 |
| B= 1992 cites to articles published in 1990-91 (this is a subset of A) |
| C= number of articles published in 1990-91 |
| D= B/C = 1992 impact factor |
The impact factor is useful in clarifying the significance of absolute (or total) citation frequencies. It eliminates some of the bias of such counts which favor large journals over small ones, or frequently issued journals over less frequently issued ones, and of older journals over newer ones. Particularly in the latter case such journals have a larger citable body of literature than smaller or younger journals. All things being equal, the larger the number of previously published articles, the more often a journal will be cited.4, 5
| Figure 2: Calculation for five-year impact factor: One year of citations to five years of articles. |
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| A= citations in 1992 to articles published in 1987-91 |
| B= articles published in 1987-91 |
| C= A/B = five-year impact factor |
An alternative five-year impact can be calculated based on adding citations in 1988-92 articles published in the same five-year period. And yet another is possible by selecting one or two earlier years as factor “B” above.
| Figure 3: Calculation for impact factor revised to exclude self-citations. |
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| A= citations in 1992 to articles published in 1990-91 |
| B= 1992 self-citations to articles published in 1990-91 |
| C= A - B = total citations minus self-citations to recent articles |
| D= number of articles published 1990-91 |
| E= revised impact factor (C/D) |
| (see Table 1 for numerical example) |