Scientific journals are ranked based on how frequently articles from the journal are cited. Ranks may be weighted based on the total number of articles in a journal or by the ranks of the journals where the citations were published. The impact factor and SCImago Journal Rank are two common ranking systems.
The impact factor is a journal rating produced by Thomson Reuters. It is calculated by finding the ratio between the number of citations received by a journal and the number of articles published in that journal over the same time period. For example, a journal that received 1000 citations but published 500 articles during the same time would have an impact factor of 2. Prestigious journals such as Science and Nature have an impact factor between 30 and 45.
The SCImago Journal Rank is calculated using information from the Scopus database. Rather than simply counting the number of citations a journal receives, it weights citations based on the rank of the journal that published them. A citation in a journal with a high SCImago rank, such as Nature or Cell, is worth far more than a citation in a lower-ranking journal. In 2014, the top 50 journals had SCImago ranks between 10,000 and 40,000. The highest-ranking journal was CA: A Cancer Journal for Clinicians, with a SCImago rank of 37,384.