Article level metrics: a look beyond the journal impact factor

The journal Impact Factor (IF), developed by Eugene Garfield at the Institute for Scientific Information (ISI), reflects the average number of times articles from the journal published in the past two years have been cited in the Journal Citation Reports (JCR) year. The Impact Factor is calculated by dividing the number of citations in the JCR year by the total number of articles published in the two previous years. For example, if there were 200 papers published in a journal in 2013 and 2014 and there were 400 citations in that time period, then the 2015 IF for the journal would be 2. Impact Factor uses Thomson Reuters (ISI Web of Knowledge) citation data. The Impact factor citation data was first derived from the Science Citation Index, a citation index created by Garfield and produced by the Institute for Scientific Information (ISI). ISI was later acquired by Thomson Reuters along with the Science Citation Index, which Reuters grew into the Science Citation Index Expanded. That index is now housed in the Web of Science, a subscription-based scientific citation indexing service encompassing six other online databases. Today, Thomson Reuters calculates IFs using the data from all of the journals indexed in the Web of Science, and releases an IF listing on an annual basis in its yearly Journal Citation Reports, which is available with paid Web of Science subscriptions