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The impact factor (also known as impact index) is an average count of how many times other authors of other journals refer to or mention the articles published in a journal during a particular period of time (usually measured during the two previous years).The formula to calculate is simple: divide the number of citations found in other journals that refer to any article of a specific journal over a certain period of time (the example shows the Journal of Investigative Dermatology) by the total number of articles published in this specific journal during the same period of time.In the specific case of the Journal of Investigative Dermatology, this calculation for the year 2001 resulted in an impact factor of 4.645. In order to calculate the impact factor, you must know the number of publications appearing in a specific journal during the previous year. That is why the impact factor is usually one year behind. For example, in 2003 we will have the factors of the year 2002; but the 2003 factors will not be known until 2004.