The impact factor (IF) of a journal measures the frequency with which its articles are cited by articles in other journals in a given year. It concerns journals in the "hard sciences", less in the "engineering sciences", very little in the "social sciences" and hardly at all in "arts and literature".

The impact factor for the year N is calculated by dividing the number of citations (during the year N) of the articles published during the years N-1 and N-2 by the number of articles published in this journal during the years N-1 and N-2.

Example: To calculate the 2012 Impact Factor of the journal Landscape Ecology
Total number of citations in 2012 of the articles published in Landscape Ecology in 2010 and 2011 = 620
Total number of articles published in Landscape Ecology in 2010 and 2011 = 214
IF 2012 = 620:214 = 2.897

The impact factor is often complemented by a 5-year impact factor that uses the same calculation method, but for data taken over the 5 previous years.

The evolution of the 5-year impact factor allows a better appreciation of the increase, the decrease or the stability of this index for the journal in question.

A journal impact factor can be compared to the impact factor of other journals in the same field. For example, Landscape Ecology can be compared to other journals in the fields it addresses: ecology, physical geography and geosciences.

The impact factor is published annually in the Journal Citation Report (JCR) by Thomson Reuters. There is always a lag between the impact factor and the year of publication: from June 2013 to June 2014, you can consult the impact factors for 2012. The impact factors for 2013 are only published in June 2014.

The JCR has incorporated an indicator created by the INRA, namely quartiles.

Since a journal can concern several fields, its impact factor should be evaluated in comparison to impact factors of journals in the same field. In a given category, it can be classified in the quarter of journals that have the highest impact factor (Q1), or in the bottom quarter (Q2) or in the second half of the journals (Q3 and Q4).

For example, the journal Landscape Ecology, which had an impact factor of 2.897 in 2012, is among the top quarter (Q1) of impact factors in the fields "Physical geography" and "Geosciences  (multidisciplinary)”, but only in the second quarter (Q2) in the field of "Ecology".

A search for the 2012 impact factor for the Landscape Ecology journal will find that this review is ranked 11th in the "physical geography" field:

The JCR provides other bibliometric indicators for journals, including:

• The Immediacy Index, which calculates the ratio between the number of citations obtained by the journal during the year N and the number of articles published by this journal during the year N. In other words, it indicates whether or not the journal articles are cited very rapidly.
• The median age of the articles cited (Cited half-life) during the year N. It indicates the number of years after which the cumulative number of citations for articles in the journal reaches 50% of the total number of citations received by the articles in the journal. For example, the median age of the journal Landscape Ecology is 7.3. This means that articles published in the journal between mid-2005 and 2012 (7.3 years) account for 50% of all citations of this journal in 2012. The index can help a library manage the archiving policy of the printed journals.

Developed by Jevin West and Carl Bergstrom at Washington University, the Eigenfactor (EF) is based on the number of citations of articles in a journal over 5 years. It takes into account the same corpus of journals as the Impact Factor for the calculation of citations.

The difference lies in the fact that it adds a weight to the citations published in journals that are themselves highly cited and it ignores self-citations of the journal.

The Eigenfactor (EF) values are normalized so that the sum of the EF values of all journals ranked in the JCR is equal to 100.

The Article Influence Score (AI) measures the average influence of journal articles over a period of 5 years after its publication. It is calculated by dividing the Eigenfactor by the fraction of the articles published in this journal, with respect to all of the articles published in all journals of the corpus.

The average influence being 1, any journal with a larger AI implies that the articles have an influence larger than the average for the journals in the category.

The Article Influence Score (AI) measures the average influence of journal articles over a period of 5 years after its publication. It is calculated by dividing the Eigenfactor by the fraction of the articles published in this journal, with respect to all of the articles published in all journals of the corpus.

The average influence being 1, any journal with a larger AI implies that the articles have an influence larger than the average for the journals in the category.

The Source Normalized Impact per Paper (SNIP) was created in 2010 by Henk Moed, at the "Centre for Science and Technology Studies" of the University of Leiden. It is based on data from Scopus (Elsevier) taken after 1996.

The SNIP is calculated by dividing the number of citations received by the articles of a journal R during the previous 3 years by the number of articles published by the journal during the same 3 years, and the citation potential for this journal in its field. The field is defined here by all the journals in which articles from journal R are cited. The citation potential is the frequency with which articles of the domain cite other articles published in the previous 3 years. The SNIP is standardized, so that a journal that is median in its disciplinary field has a citation potential of 1.

Unlike the Impact Factor, this indicator is freely accessible: http://www.journalindicators.com/indicators

The SJR (SCImago Journal Rank) indicator, like the journal impact factor, is based on the number of citations received by the articles of a journal. Its calculation is more complex because it weights each citation by the "prestige" of the citing journal (without specifying how this is done) and does not take into account the self-citations of the journal. http://www.scimagojr.com/SCImagoJournalRank.pdf

This indicator is freely accessible: http://www.scimagojr.com/

Created in 2010 by González-Pereira, Guerrero-Bote and Moya-Anegon of the University of Granada (Spain), the SJR is based on the journals in Scopus (Elsevier) and thus on a much larger number of journals than the Impact Factor. The journals in the SHS are better represented, even if, as for the Impact Factor, there is an overrepresentation of journals from the Anglo-Saxon world.