The Impact Factor of a Journal

8 downloads 130 Views 1MB Size Report
The Impact Factor of a Journal. Prof. Mazin M. Elias. Institute of Laser for Postgraduate Studies. University of Baghdad. December 2012 ...

The Impact Factor of a Journal Prof. Mazin M. Elias Institute of Laser for Postgraduate Studies University of Baghdad

December 2012

OUTLINE  Impact Factor; Background; Calculation

     

Notes associated with the use of journal IF. The citation curve. Parameters affecting the impact factor. Comparison of different journal ranking systems A new journal ranking system. Conclusions

M.M. Elias . The Impact Factor of a Journal, 2012

Impact Factor (IF) • A factor to approximate which journals have the greatest impact. It indicates the relative significance and influence of a particular journal within its field of research. We use IFs to select the best journals to submit our research for publication. • IFs are calculated yearly, and published in the annual Journal Citation Reports (JCR). This publication lists the IFs for all journals (over 13000) indexed in the ISI Science Citation Index, and groups journals by discipline. M.M. Elias . The Impact Factor of a Journal, 2012

Historical Background of Impact Factor

>

Historical Background of Impact Factor

Eugene Garfield, Thomson Reuters Founder

Citation Indexes for Science: A New Dimension in Documentation through Association of Ideas"

Science (July 1955)

• In England, hiring panels routinely consider impact factors. • By Spanish law, researchers are rewarded for publishing in journals defined by ISI as prestigious (upper third of impact factor listings). • In Iraqi and China, scientists get cash bonuses for publishing in high-impact journals. • In some colleges, physics students must publish at least 2 articles with a combined Impact Factor of 4 to get their PhD. M.M. Elias . The Impact Factor of a Journal, 2012

Calculation of Impact Factor Impact Factor is considered to be the average number of times published papers are cited up to two years after publication, e.g., the impact factor 2012 would be calculated as follows: A = number of times articles published in 2010-2011 were cited in indexed journals during 2012 B = number of articles, reviews or proceedings published in 2010-2011 Impact Factor 2012 = A/B Note that IF2012 is actually published in 2013, because it could not be calculated until all of 2012 publications had been received. M.M. Elias . The Impact Factor of a Journal, 2012

Example …. 2012 A 2011.

A A 2012

A

….

2010

Thomson Reuters ISI ranking for academic journals

Thomson Reuters, has provided access to academic research information for over 55 years following the work of its founder Dr. Eugene Garfield. ISI is the most respected ranking for academic journals in the world, and is used for key decisions such as whether an author will publish with a journal, and whether a library will subscribe to it.

M.M. Elias . The Impact Factor of a Journal, 2012

General Notes • IF represents an arithmetic mean, rather than a normal distribution; some believe that this fact makes the IF unfit for evaluating relative impact of a journal. • Review articles generally are cited more frequently than typical research articles because they often serve as substitutes for earlier literature. In the JCR system any article containing over 100 references is coded as a review. • Publishing in English is an indicator that the publisher recognizes that the maximum number of readers can be reached with English. M.M. Elias . The Impact Factor of a Journal, 2012

Notes associated with the use of journal impact factors • Journal IFs are not statistically representative of individual journal articles. They correlate poorly with actual citations of these articles. • Self citations are allowed. • Biased toward English-language journals – ISI has recently added several hundred non-English journals

• Long or review articles collect many citations and give high journal impact factors.

M.M. Elias . The Impact Factor of a Journal, 2012

Notes associated with the use of journal impact factors • Coverage of the database is not complete (13000 out of 128000 journals). Books are not included in the database as a source for citations.

• Short (two year) snapshot of journal – Some disciplines use older material more or take time to cite new research – JCR now also includes the 5-year data

M.M. Elias . The Impact Factor of a Journal, 2012

Notes associated with the use of journal impact factors • Journal IFs depend on the research field. The citation impact of a research field is directly proportional to the mean number of references per article, which varies considerably from field to field (e.g., it is twice as high in biochemistry as in mathematics). • Small research fields tend to lack journals with high impact

M.M. Elias . The Impact Factor of a Journal, 2012

Notes associated with the use of journal impact factors • In a largely self contained research field, the mean article (or journal) citation rate is independent of the size of the field, but the absolute range will be wider in a large field. • Article Influence scores are normalized so that the mean article in the entire Thomson Journal Citation Reports (JCR) database has an article influence of 1.00. A score greater than 1.00 indicates that each article in the journal has above-average influence.

M.M. Elias . The Impact Factor of a Journal, 2012

Notes associated with the use of journal impact factors • Although IF remains the most frequently used number for assessing journals’ influence, other similar calculations exist, e.g., H-Index (which focuses on assessing the work of a particular scholar) uses a formula that includes total output and citations for a specific researcher, and Eigen factor (provides slightly different metrics from IF as significance of citations is considered.

M.M. Elias . The Impact Factor of a Journal, 2012

Self-citation in Impact Factor Cited-Only Journals in the JCR Some journals listed in the JCR are not citing journals, but are cited-only journals. This is significant when comparing journals by impact factor because the self-citations from a cited-only journal are not included in its impact factor calculation. Self-citations represent about 13% of the citations.

Calculation for impact factor revised to exclude self-citations A= citations in 2012 to articles published in 2010-2011 B= 2012 self-citations to articles published in 2010-2011 C= A - B = total citations minus self-citations to recent articles D= number of articles published 2010-2011 E= revised impact factor (C/D) M.M. Elias . The Impact Factor of a Journal, 2012

The citation curve of a journal • Impact Factor is only one of three standardized measures created by the Institute of Scientific Information (ISI) used to measure the way a journal receives citations to its articles.

• The build-up of citations tends to follow a curve. Citations to articles published in a given year rise sharply to a peak between 2 and 6 years after publication. • The citation curve of any journal can be described by the relative size of the curve (area under the line), the extent to which the peak of the curve is close to the origin, and the rate of decline of the curve. These characteristics form the basis of the ISI indicators impact factor, immediacy index and cited half-life. M.M. Elias . The Impact Factor of a Journal, 2012

The citation curve ….

The immediacy index The immediacy index gives a measure of the skewness of the curve, that is, the extent to which the peak of the curve lies near to the origin of the graph. It is calculated by dividing the citations a journal receives in the current year by the number of articles it publishes in that year, i.e., the 2012 immediacy index is the average number of citations in 2012 to articles published in 2012.

The number that results can be thought of as the initial gradient of the citation curve, a measure of how quickly items in that journal get cited upon publication. M.M. Elias . The Impact Factor of a Journal, 2012

The cited half-life The cited half-life is a measure of the rate of decline of the citation curve. It is the number of years that the number of current citations takes to decline to 50% of its initial value (the cited half-life is 6 years in the example given in Figure). It is a measure of how long articles in a journal continue to be cited after publication.

M.M. Elias . The Impact Factor of a Journal, 2012

Parameters affecting the impact factor The value of the impact factor is affected by sociological and statistical factors. Sociological factors include • Subject area of the journal • Type of journal (letters, full papers, reviews) • Average number of authors per paper (which is related to subject area).

Statistical factors include • Size of the journal • Size of the citation measurement window. M.M. Elias . The Impact Factor of a Journal, 2012

1. Subject Variation in Impact factor ….

The figure shows how the value of the mean IF exhibits significant variation according to

subject field. Fundamental and pure subject areas have higher average IF than specialized or applied ones. The variation is so significant that the top journal in one field may have an impact factor lower than the bottom journal in another area.

2. Impact Factor and Number of Authors per Paper

….

The average number of authors in a paper varies according to subject area.

There is a significant correlation between the

average number of authors per paper and the average impact factor for a subject area . So comparisons of impact factors should only be made for journals in the same subject area.

3. Impact Factor with Article and Journal Type Even within the same subject there will be significant variation according to the journal or article type. A short or rapid publication journal (“Letters” journal) publishing short papers, will have greater immediacy but a lower cited half-life (the peak of the citation curve is closer to the origin and the curve will turn down rapidly).

4. Impact Factor Fluctuation vs. Journal Size IF is an average value, thus it shows statistical variations. These relate to the number of items being averaged, that is the size of the journal in terms of articles published per year, or the size of the window. There is a clear correlation between the extent of the IF fluctuation and the size of the journal. When IFs are compared between years, size of the journal should be considered. Small titles (less than 35 papers /y) vary in IF by over + 40% from one year to the next. M.M. Elias . The Impact Factor of a Journal, 2012

….

….

5. Impact Factor Fluctuation vs Measurement Window Size Expanding the size of the measurement window from the two years of the standard JCR impact factor can iron out some of the statistical variations.

….

….

….

….

JCR Home Page

Science and Social Science editions must be searched separately

Before starting, click on Information for New Users and read “Using the JCR Wisely.”

Journal Search Screen

You can search by Full Journal Title, Journal Abbreviation, Title Word, or ISSN. Select Title Word from the menu.

2010 Journal Citation Reports in the field of Molecular Biology sorted by Impact Factor (2-year)

Impact factors are listed in Journal Citation Reports JCR

Comparing JCR and Eigenfactor Journal

5-year JCR Impact Factor

Eigenfactor Article Influence Score

Nature Rev Molec Cell Biol

35.423 (100)*

19.970 (100)

Cell

31.253 (88)

18.871(94)

Nature Medicine

27.553(78)

12.958(65)

Ann Rev Cell Biology

22.731(64)

16.220(81)

Nature Cell Biology

17.774(50)

10.872(54)

Cell Stem Cell

16.826(48)

12.304(62)

Cell Metabolism

16.107(45)

9.506(48)

*In parenthesis, values normalized.

A New Journal Ranking System SCImago Journal Rank (SJR) • The citation PageRank of a journal calculated on the basis of the Scopus citation data divided by the number of articles published by the journal over 3 years. • Similar to Eigenfactor methods, but based on citations in Scopus instead of Web of Science.  Freely available at scimagojr.com  Covers more journals (~20,000) than Web of Science  More international range  3 years of citations; no self-citations M.M. Elias . The Impact Factor of a Journal, 2012

Comparison of Three Journal Ranking Systems Data of 2007 JCR Journal Impact Factor (2 year)

JCR Journal Impact Factor (5 year)

Eigenfactor Article Influence Score (5 year)

SCImago Journal Rank (3 year)

Science

26.372

30.631

16.539

3.726

Cell

29.887

28.779

18.188

10.735

Nature

28.751

28.751

16.996

4.636

PNAS

9.598

10.369

4.929

2.689

BMC Bioinformatics

3.493

4.221

1.608

0.750

Bioinformatics

5.039

6.649

2.406

1.225

Journal

M.M. Elias . The Impact Factor of a Journal, 2012

Conclusions • The IF value is affected by the subject area, type and size of a journal, and the “window of measurement” used. • As statistical measures they fluctuate from year to year, so that great care needs to be taken in interpreting whether a journal has really “dropped (or risen)” in quality from changes in its IF. • Use of the absolute IF values outside of the context of other journals within the same subject area, is virtually meaningless; journals ranked top in one field may be bottom in another. M.M. Elias . The Impact Factor of a Journal, 2012

Conclusions • Extending the use of the journal IF from the journal to the authors of papers in the journal is highly suspect; the error margins can become so high as to make any value meaningless. Professional journal types frequently contain many more types of source item.

• To increase IF of a journal, the editor must be able to attract authors who publish original and innovative research that captures the attention of the international scientific community. To maximize the impact one needs establish high standards and avoid becoming a dumping ground for secondrate or lower research. Use an international board of editors.

M.M. Elias . The Impact Factor of a Journal, 2012

“What matters absolutely is the scientific content of a paper, and nothing will substitute for either knowing or reading it” Sidney Brenner, Nature 1995

38