The Institute of Scientific Information has edited an index to evaluate the diffusion of scientific articles. It is based on the fact that the more an article is cited as a reference in other articles, the more it is considered to be important, and the higher the coefficient attributed to the journal in which it was originally published. Several methods of evaluation have been proposed, the two main ones being the impact and citation half-life. The impact factor takes into account the average number of times which a journal is mentioned for recent articles published in a given year. The citation half-life gives an insight into the longeivity of the articles of a journal by the number of citations received. This index is an attempt to quantify the notoriety of scientific journals for all scientific medical specialties. Each year, the institute for Scientific Information publishes a classification of the journals of each medical specialty.