Our unique Research and Academic Relations Department has a great deal of experience and knowledge in the use and limitations of bibliometric journal measures. Upon request by your publishing contact, we can provide data and analysis to highlight areas in which a journal excels and in which is can focus to improve its performance.
Impact Factor
The Journal Impact Factor is published each year by Thomson Reuters. It is a measure of the number of times an average paper in a particular journal is referred to.
The Impact Factor of journal J in the calendar year X is the number of citations received by J in X to any item published in J in (X-1) or (X-2), divided by the number of source items published in J in (X-1) or (X-2).
'Source items' is the term used to refer to full papers: original research articles, reviews, full length proceedings papers, rapid or short communications, and so on. Non-source items, such as editorials, short meeting abstracts, and errata, are not counted in the denominator although any citations they might receive will be included in the numerator.
An example follows for the fictitious Journal of Great Science:
* In year X, the Journal of Great Science received 152 citations to items published in (X-1) and 183 citations to items published in (X-2). Total citations for Impact Factor calculation = 335.
* 123 source items were published in the Journal of Great Science in (X-1), and 108 in (X-2). Total source items for Impact Factor calculation = 231.
* Year X Impact Factor for the Journal of Great Science = 335/231 = 1.450.
Impact Factor can be affected by subject field, number of authors, content type, and the size of the journal; this is described in our Perspectives in Publishing paper, from which the figure above, showing a generalized citation curve and how Thomson Reuters’ metrics relate to it, is taken.
The Impact Factor can be a useful way of comparing citability of journals, if the comparison is limited to a given subject field and the type of journals being compared (review, original research, letters) are similar. The absolute Impact Factor is of limited use, without that of other journals in the field against which to judge it.
A new indicator called SNIP (Source Normalized Impact per Paper) has recently been developed by Hank Moed who was then part of the CWTS bibliometrics group at the University of Leiden. The pre-calculated metric is freely available to all at http://info.scopus.com/journalmetrics/ and early 2010 was added to the Scopus Journal Analyser.
SNIP is a novel approach and as such provides a novel bibliometric perspective. Key points include:
Freely available on the web.
Available in Scopus, alongside SJR.
Calculated for more journals than the Impact Factor.
Updated twice a year.
SNIP is defined as the ratio of the Raw Impact per Paper divided by the Relative Database Citation Potential. The Raw Impact per Paper is the ratio of citations in year X to peer-reviewed papers published in years X-1, X-2 and X-3 divided by the number of peer-reviewed papers published in years X-1, X-2 and X-3. As such it is conceptually similar to the Impact Factor. For example, the 2009 SNIP is citations made in 2009 to peer-reviewed papers published in 2006, 2007 and 2008, divided by the number of peer-reviewed papers published in 2006, 2007 and 2008, this ratio being in turn divided by the Relative Database Citation Potential. Detailed methodology can be found here.
Five year Impact Factor
The five year impact factor is similar in nature to the JCR Impact Factor, but instead of counting citations in 2008 to the previous two years and dividing by source items in these years, citations are counted in 2008 to the previous five years and again divided by the source items published in the previous five years.
An example for Tetrahedron Letters:
2 yr Impact Factor: 10316 citations in 2007 to items published in 2005 and 2006/ 3945 items published in 2005 and 2006= 2.615
5 yr Impact Factor: 25378 citations in 2007 to items published in 2006, 2005, 2004, 2003, and 2002/ 10446 items published in 2002-2006 = 2.429
A base of five years may be more appropriate for journals in certain fields because the body of citations may not be large enough to make reasonable comparisons or it may take longer than two years to disseminate and respond to published works. The two measures differ also in the amount of variability between years. The two year Impact Factor can fluctuate by around 20% in value each year, whereas the five-year measure, while still showing changes over time, presents a much smoother variation.
The exact number in the metric may differ, but often this difference disappears when one looks at the relative position of a journal within its subject field. If the whole field evolves slower and benefits from a 5 yr measure, the rankings will not differ much.
Ranking
Journals are often ranked by impact factor in an appropriate ThomsonReuters subject category. As there are now two published impact factors, this rank may be different when using a two or a five year impact factor and care is needed when assessing these ranked lists to understand which metric is being utilised. In addition journals can be categorised in multiple subject categories which will cause their rank to be different and consequently a rank should always be in context to the subject category being utilised.
Immediacy Index
Thomson Reuters publish other metrics, in addition to the Impact Factor. The Immediacy Index is a measure of the speed at which content in a particular journal is picked up and referred to, and is illustrated in the figure above.
The Immediacy Index of journal J in the calendar year X is the number of citations received by J in X to any item published in J in X, divided by the number of source items [link to paragraph in Impact Factor section] published in J in X.
An example follows for the fictitious Journal of Great Science:
* In year X, the Journal of Great Science received 84 citations to items published in X.
* 120 source items were published in the Journal of Great Science in X.
* Year X Immediacy Index for the Journal of Great Science = 84/120 = 0.700.
Like the Impact Factor, the Immediacy Index can be affected by characteristics peculiar to the particular field. It will only be important for those fields in which citations start to flow in quite quickly, such as fundamental life sciences or neurosciences.
Cited Half-Life
Thomson Reuters also publish the Cited Half-Life, in addition to the Impact Factor and Immediacy Index. The Cited Half-Life is a measure of the ‘archivability’ of content in a particular journal, or of how long content is referred to after publication; it is illustrated in the figure above.
The Cited Half-Life of journal J in year X is the number of years after which 50% of the lifetime citations of J’s content published in X have been received.
Like the Impact Factor and Immediacy Index, the Cited Half-Life can be affected by characteristics peculiar to the particular field. It will be more important for those fields in which citations start to flow in slowly after a significant lag time, such as social sciences, or mathematics and computer sciences.
Journal h-index
The h-index was proposed in 2005, by Professor Jorge Hirsch, as a metric for evaluating individual scientists; the paper is freely available.
However, the h-index can be applied to any group of articles, including those published in a particular journal in any given year.
If a set of papers is arranged in descending order of the lifetime citations received, the h-index h is the highest number for which it is true to say that h articles have each received at least h citations.
In the fictitious example below, the 80 articles published in a journal in a given year have been ranked by lifetime citations. The h-index of this journal for this year’s content is 22, since 22 articles have each received at least 22 citations.
Since the original paper proposing the h-index, several variants have been proposed. One of these is the g-index, developed by Professor Leo Egghe, which aims to emphasise the contribution of the top papers that can have many more citations than the h-index would suggest. The g-index is the highest number of papers for which it is true to say that g papers together received at least g2 citations. This means that the g-index will always be higher than the h-index.
Articles on the evolution of the h-index, published in our free e-newsletter Research Trends (www.researchtrends.com), are available:
The SCImago Journal Rank was recently developed by SCImago, a research group from the University of Granada, Extremadura, Carlos III (Madrid) and Alcalá de Henares, dedicated to information analysis, representation and retrieval by means of visualization techniques.
SCImago Journal Rank is based on citation data of the more than 15,000 peer-reviewed journals indexed by Scopus from 1996 onwards, and is freely available at www.scimagojr.com.
The SCImago Journal Rank of journal J in year X is the number of weighted citations received by J in X to any item published in J in (X-1), (X-2) or (X-3), divided by the total number of articles and reviews published in (X-1), (X-2) or (X-3).
SCImago Journal Rank is a measure of the number of times an average paper in a particular journal is referred to, and as such is conceptually similar to the Impact Factor. A major difference is that instead of each citation being counted as one, as with the Impact Factor, the SCImago Journal Rank assigns each citation a value greater or less than one based on the rank of the citing journal. The weighting is calculated iteratively from an arbitrary constant using a three-year window of measurement. Detailed methodology can be found at www.scimagojr.com/SCImagoJournalRank.pdf.
Eigenfactor and Article Influence
The Eigenfactor and Article Influence are recently developed metrics based on data held in Thomson Reuters’ Journal Citation Reports. They are freely available at www.eigenfactor.org.
The Eigenfactor of journal J in year X is defined as the percentage of weighted citations received by J in X to any item published in (X-1), (X-2), (X-3), (X-4), or (X-5), out of the total citations received by all journals in the dataset. Only citations received from a journal other than J are counted. The Eigenfactor is not corrected by article count, and so is a measure of the influence of a particular journal; bigger and highly cited journals will tend to be ranked highly.
As with the SCImago Journal Rank, each (non-self) citation is assigned a value greater or less than one based on the Eigenfactor of the citing journal. The weighting to be applied is calculated iteratively from an arbitrary constant. Detailed methodology can be found at www.eigenfactor.org/methods.htm.
Article Influence is calculated by dividing the Eigenfactor by the percentage of all articles recorded in the Journal Citation Reports that were published in J. Article Influence is therefore is conceptually similar to the Impact Factor and SCImago Journal Rank.