The impact factor is calculated by dividing the number of times articles from a journal are cited in a core of about 5000 journals by the number of articles published by that journal in the same period of two years. It is used as a measure of a journal's scientific influence, but the assumptions underlying this use are flawed. Although the use of the impact factor is recognized to be invalid, nonetheless it continues, in part because no better agreed measure has yet been developed.
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