The impact factor is a measure of the frequency with which the "average article" in a journal has been cited in a particular year or period.
JCR® is the sole place where impact factors are calculated and reported. JCR® covers approximately 7,500 journal titles. This is a small minority of the journals in publication, so be aware: most journals do not have an impact factor.
ISI defines the impact factor as a ratio of two elements. The denominator is the total number of "citable" articles published in a particular journal within a given timeframe. The numerator is the total number of citations in the current year to any article published in this journal during that given timeframe. ISI has defined this time frame as two years.
The impact factor (IF) of journal A in a particular year Y is calculated using the following formula:
Impact factors can be used to:
But what’s in a number? To state that the British Medical Journal (BMJ) has an impact factor of 9.052 is reasonably meaningless.
It is more meaningful to say that BMJ’s impact factor ranks sixth of 105 journals in the field of general and internal medicine. Or to compare the journal’s impact factor of 9.052 with the aggregate impact factor for its field: 4.326.
It is recommended therefore that the impact factor for a journal is not looked at in isolation. Rather, the impact factor of a journal should be compared to the impact factors for other journals within the same subject category.
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