An Impact Factor is a quantitative measure of the relative importance of a journal, individual article or scientist to science and social science literature and research.
Each index or database used to create an impact factor uses a different methodology and produces slightly different results. This is why it is important to use several sources to gauge the true impact of a journal's or scientist’s work.
Informed and careful use of these impact data is essential, and should be based on a thorough understanding of the methodology used to generate impact factors. There are controversial aspects of using impact factors: