Journal performance

This page is intended to provide you with definitions of bibliometric indicators used to measure the influence of journals, as well as to guide you on their uses and limitations.

On the first tab we discuss the Impact Factor, which remains the bibliometric indicator most commonly used by researchers and research management.

However, criticism of the Impact Factor has led to the development of new initiatives and indicators. On tab two, we explain the other metrics included in Thomson Reuters’ Journal Citation Reports, such as the Immediacy Index, Cited Half-life and Eigenfactor.

Scopus has also selected bibliometric indicators to measure the influence of journals. On tab three, you can read about SNIP (Source Normalized Impact per Paper) and SJR (SCImago Journal Rank). We also examine the h-index, which is a measure of an individual’s performance.

On the journal homepage the Journal Insights pod gives a powerful visualization of five years of historical data. It takes into account eight metrics available within three key groups (quality, speed, authors) developed to aid your decision making regarding which journal to submit to. 

Impact Factor

The Journal Impact Factor is published each year by Thomson Reuters. It measures the number of times an average paper in a particular journal has been 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).

Journal PerformanceSource items

'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.

You can find the most recent Impact Factors of our individual journals on their homepages.

Five-year Impact Factor

The five-year Impact Factor is similar in nature to the regular ‘two-year’ Impact Factor, but instead of counting citations in a given year to the previous two years and dividing by source items in these years, citations are counted in a given year 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: 9621 citations in 2010 to items published in 2008 and 2009 / 3675 items published in 2008 and 2009 = 2.618
5-yr Impact Factor: 23846 citations in 2010 to items published in 2005, 2006, 2007, 2008, and 2009 / 9602 items published in 2005-2009 = 2.483

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 Thomson Reuters 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 utilized. In addition, journals can be categorized 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 utilized.

Further metrics

Journal PerformanceImmediacy 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 on the right.

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 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 the 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.

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. See detailed methodology.

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 conceptually similar to the Impact Factor and SCImago Journal Rank.

Metrics in Scopus

Source Normalized Impact per Paper

An indicator called SNIP (Source Normalized Impact per Paper) was developed by Henk Moed who was then part of the CWTS bibliometrics group at the University of Leiden. The pre-calculated metric was added to the Scopus Journal Analyzer in early 2010 and is freely available at www.journalmetrics.com.

SNIP is a novel approach and as such provides a novel bibliometric perspective. The key idea behind SNIP is that it corrects for subject-specific characteristics of the field someone is publishing in. This means that, contrary to the Impact Factor, SNIP numbers can be compared for any two journals, regardless of the field they are in.

Additional points include:

  • Freely available on the web.
  • 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 2010 SNIP is citations made in 2010 to peer-reviewed papers published in 2007, 2008 and 2009, divided by the number of peer-reviewed papers published in 2007, 2008 and 2009, this ratio being in turn divided by the Relative Database Citation Potential. See detailed methodology.

SCImago Journal Rank

The SCImago Journal Rank (SJR) 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 18,000 peer-reviewed journals indexed by Scopus from 1996 onwards, and is freely available at www.scimagojr.com.

The central idea of SJR is that citations are weighted, depending on the rank of the citing journal. A citation coming from an important journal will be counted for more than one citation, a citation coming from a less important journal will be counted for less than one citation.

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.
See detailed methodology.

Additional information on SJR and a data file containing all SJR values can be found at
www.journalmetrics.com.

h-index

The h-index was proposed in 2005 by Professor Jorge Hirsch, as a metric for evaluating individual scientiststhe paper is freely available.

The h-index rates a scientist's performance based on his or her career publications, as measured by the lifetime number of citations each article receives. The measurement is dependent on both quantity (number of publications) and quality (number of citations) of an academic's publications.

If you list all of a scientist's publications in descending order of the number of citations received to date, their h-index is the highest number of their articles, h, that have each received at least h citations. So, their h-index is 10 if 10 articles have each received at least 10 citations; their h-index is 81 if 81 articles have each received at least 81 citations. Their h-index is one if all of their articles have each received one citation, but also if only one of all their articles has received any citations.

However, the h-index can be applied to any group of articles, including those published in a particular journal in any given year.

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.

Article rank

1

2

3

21

22

23

78

79

80

Citations to date

72

63

59

24

24

21

0

0

0

Resources
www.journalmetrics.com
White Paper on the h-index and Scopus
An article on the h-index published in Editors’ Update in September 2007