What is the Impact Factor?
The Impact Factor (or IF) continues to play a prominent role in the ranking of academic journals in many fields. In essence, the Impact Factor simply denotes the average citations to recent articles published in a particular journal, but in enabling a quantitative comparison between competing journals it is also frequently – but not always accurately – used as a short-hand for measuring and comparing ‘quality’.
How is it calculated?
An Impact Factor is assigned to journals indexed in the Thomson Reuters Web of Science, and is calculated as the number of citations received by one journal from other indexed journals in one year to articles published by that same journal in the previous two years (see figure above). The temporal component of the calculation prevents newer journals receiving a full Impact Factor, however influential their articles within a given research ecosystem.
According to Open Science, the “Impact Factor (IF) is the oldest, most renowned and extensively used index for measuring the quality of a journal, but … it does have its flaws and can be manipulated, so it needs to be treated with some precautions.” 
Citations denoting impact
The Impact Factor was created by Eugene Garfield (founder of the Institute for Scientific Information, now Thomson Reuters Web of Science), and designed primarily to aid comparison of journals in particular academic fields. However, over the years the concept has often been more broadly interpreted and applied, with higher citation counts often becoming synonymous with greater impact and quality.
Thomson Reuters echoes the views of many researchers as regards citations and impact, that a high citation count is indicative of “the influence or impact of the idea and its originator on our body of knowledge.”  High citation is undoubtedly one important component of research impact, implying visibility and endorsement for the work among one’s peers – supporting that principle of scholarly activity, building on the shoulders of giants. In this light, it is perhaps no surprise that a high Impact Factor has come to convey prestige on a journal, making the prospect of publishing within a high-impact journal appealing to researchers.
Nevertheless, there is a growing recognition that understanding the impact of individual articles is not satisfactorily addressed by the Impact Factor, as impact (in the form of citations) is calculated only at the aggregated level of the entire journal. For instance, Philip Campbell, editor-in-chief of Nature, has written that “89% of our Impact Factor was generated by just 25% of our papers”  on analysing citations in 2004 of papers published between 2002 and 2003. Another study in 1992 from Per O. Seglen revealed the same: an extremely small proportion of individual journal articles can account for journal citations, accounting for overall journal impact. 
Thomson Reuters also writes of the limitations and challenges that measuring impact entails when considering journals in different academic disciplines, as well as the fact that new journals cannot immediately gain a calculated Impact Factor:
“Using quantitative citation data to measure impact is meaningful only in the context of journals in the same general discipline. For example, smaller fields … do not generate as many articles or citations … in some areas, particularly in the Arts & Humanities, it may take a relatively long time for an article to attract a meaningful number of citations. … For new journals that do not yet have a citation history at the publication level, analysts examine the citation record of the contributing authors and editorial board members. This allows them to see if the journal is able to attract contributions from scholars whose prior work has been useful to the research community.” 
Consequently, all new journals, such as those in the Cogent OA series, require a period of at least two years before they become eligible for an Impact Factor. Even after this point, a more granular analysis of citations is required to more fully understand impact at the article level.
To address some of these limitations, Cogent OA is developing a host of article-level metrics for authors who publish in one of our journals that are intended to complement traditional journal-level metrics. These article-level metrics are assigned to an article after publication in order to illustrate the evolving impact of each individual article over time.
 Falkowska, Marzena. 2012. 'Impact Factor - How To Get | Openscience.Com'. Open Science. http://openscience.com/how-to-get-impact-factor/.
 Thomson Reuters. 2015. 'History Of Citation Indexing - IP & Science - Thomson Reuters'. Wokinfo.Com. http://wokinfo.com/essays/history-of-citation-indexing/.
 Campbell, Philip. 2008. 'Escape from the impact factor'. Ethics in Science and Environmental Politics. http://www.int-res.com/articles/esep2008/8/e008p005.pdf.
 Seglen, Per O. 1992. 'The Skewness Of Science'. Journal Of The American Society For Information Science 43 (9): 628-638. http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1097-4571(199210)43:9%3C628::AID-ASI5%3E3.0.CO;2-0/abstract.
 Thomson Reuters. 2015. 'The Thomson Reuters Journal Selection Process - IP & Science - Thomson Reuters'. Wokinfo.Com. http://wokinfo.com/essays/journal-selection-process/.