The 1st scientific journals, French Journal des sçavans and the English Philosophical Transactions of the Royal Society in London, were published in the middle of 1600. By the middle of the 18 century, there were only 10 journal existed. When there were few journals, it was easy to evaluate and use scientific contents. However, numbers of journals are now increasing so fast that there is a need of a tool to compare among journals.
An idea of an impact factor was first mentioned in Science magazine in 1955. It has evolved over time. The impact factor of a journal now is calculated based on two elements. First, the numerator is the number of citation in the current year to any items published in the journals in the previous 2 years. Second, the denominator is the number of substantive articles (source items) published in the same 2 years. In other words, it is a measurement of the frequency that the average article in a journal has been cited in a particular period . Therefore, the journal impact factor shows the importance of absolute citation frequencies. This measurement overcomes some biases of comparing large journals over small ones, frequently issued journals over less frequently issued ones, or older journals over new ones. It also receives numerous criticisms. For example, I agree that in some journals just a small portion of influential articles get most of the citations, while a much larger portions received few or even none at all. Therefore, the average number of citation might be misleading. Another thing is who cited those papers.
For individuals, the impact factor has been used as an approximation of the prestige of journals for deciding where to publish their work. It also influences assessment of who gets jobs, tenure, and grants. However, a number alone do not tell the real story. To be more accurate, it should be considered in conjunction with other factors such as peer review, productivity, and subject specialty citation rates .
I agree that value of a scientific community cannot be captured in a single number, but the journal impact factor is still a useful tool. We might have to use it together with other factors to have an overall picture.
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