Journal-level metrics attempt to quantify a journal's impact, by analyzing (in different ways and over different amounts of time) how frequently its articles are cited.
Pros: These metrics can give a sense of which journals are popular and/or respected within a specific field.
Cons: These metrics effectively average the impact of a journal's articles and authors, so they hide variation in citedness between articles and authors. They also are not generalizable across disciplines.
Google Scholar Metrics is available by clicking "Metrics" from the Google Scholar homepage. It offers a list of the top journals in specific fields according to their h-index and h5-median values, based on the citation information in Google Scholar.
Journal Citation Reports (JCR) is the journal-level metrics database on the Web of Science platform. It includes:
Impact Factor is the number of citations to a specific journal's articles in a given year, divided by the number of articles published by that journal over the two years prior. For example, a current Impact Factor of 2.5 means that, on average, the articles published one or two years ago have been cited two and a half times.
5-year Impact Factors are often calculated as well. When this same calculation is made over 1 year, that number is called the Journal Immediacy Index. Impact Factors may include citations between articles in the same journal.
A journal's Eigenfactor Score is calculated using a complex algorithm. Journal self-citations are excluded, and citations are iteratively weighted by the citing journals Eigenfactor Score (compare to Google's PageRank, where being linked by "more authoritative" websites causes results to appear higher in search results).
An Article Influence Score can then be obtained by averaging a journal's Eigenfactor Score over the number of articles published.
SCImago Journal Rank (SJR) is freely available online, and depends on an iterative algorithm where citations are weighted more or less based on the journal they arise from. SJR uses data from the Scopus database.
Journal metrics and rankings are not synonymous with journal quality. To get a fuller picture of a journal's quality, researchers should examine the journal itself and think critically about its policy, leadership, articles, etc. The site Think. Check. Submit. lists questions that researchers should ask themselves when evaluating a journal, especially when considering whether to publish with that journal.