A measure of impact.
The time period used in a citation analysis. Often a two-year interval, e.g. for an article published in 2009, citations from 2010 and 2011 are counted.
A qualitative bibliometric indicator. A field-normalised mean. The number of citations received during a defined period of time divided by the world average for the same discipline, same time period and same type of publication. The world average = 1.0.
Dividing up the number of citations received by a paper by the number of authors. This can also be done according to the number of institutions, countries and so on in the author address fields.
Hirsch index (h-index)
A quantitative bibliometric indicator for authors. An author′s h-index is the number of publications (h) by the author which have been cited at least h number of times. E.g. if an author has published 53 articles, of which 15 have been cited 15 or more times, that author′s h-index is 15.
A qualitative bibliometric indicator for journals (not for authors or articles). IF shows how many times the average article in a journal is cited. Journals with the highest impact factors have one of about 50, while most are below 1. The impact factors of journals in different fields should not be compared with each other.
The impact factor 2,5 for the year 2011 means: The journal’s articles in 2009 and 2010 had approximately 2,5 citations during the year 2011.
There is also a five-year impact factor, which is a more reliable indicator in many subject areas. Articles from the years 2010, 2009, 2008, 2007, 2006 are included in the five-year impact factor for 2011.
The impact factor is often misused in measuring quality. The common assumption is that more cited journals have more rigorous standards for acceptance, but in truth about 20% of the articles provide 80% of the citations even in journals with higher impact factors. Most articles have low citation values even though they are published in a highly ranked journal. The journal impact factor does not say anything certain about the quality of the individual articles.
Comparing like with like. Field normalisation, for example, means that the number of citations received by an author for articles within a certain discipline are compared with the world average (usually the Web of Science average) for that discipline. A score of >1.0 means the author has a citation per paper score better than the world average.
When an author cites a work she⁄he has co-authored. Bibliometric analyses often choose not to include self-citations.
A quantitative bibliometric indicator for publications. The proportion of publications among the 5% most cited publications in that subject area.