In the natural sciences and in the social sciences, the impact factor is an established proxy, measuring the number of later articles citing articles already published in the journal. There are other quantitative measures of prestige, such as the overall number of citations, how quickly articles are cited, and the average "half-life" of articles. Clarivate Analytics' "Journal Citation Reports", which among other features, computes an impact factor for academic journals, draws data for computation from the Science Citation Index Expanded (for natural science journals), and from the Social Sciences Citation Index (for social science journals). Several other metrics are also used, including the SCImago Journal Rank, CiteScore, Eigenfactor, and Altmetrics.