What does impact factor depend upon?

Nobody doubts the importance of the scientific performance evaluation. The journal impact factor is increasingly employed to evaluate the quality of scientific research. The use of term "impact factor" has gradually evolved, especially in Europe, to include both journal and author impact. This ambiguity often causes problems. It is one thing to use impact factors to compare journals and quite another to use them to compare authors. Journals impact factors generally involve relatively large numbers of articles and citations. Individual authors, on average, produce much smaller numbers of articles. Many scientists consider that impact factor is not the perfect tool to measure the quality of articles but there is nothing better and it has the advantage of already being inexistence and is, therefore, a good technique for scientific evaluation. However the use of journal impact factor is probably the most controversial issue.