Circulation – number of
copies per month?
Readership – many readers
Regular readers – browse
Referred to by researchers
or experts – specific articles
located by literature search
1955 - Eugene Garfield in Science - Citation is key
1961 – Institute for Scientific Information - Science citation index
1992 – bought byThomson Scientific – nowThomson-Reuters
Impact factor =
Citations in current year to articles in previous 2 years
Number of articles published in previous 2 years
Journal impact factor =
Number of articles (2007,2008) from the journal cited by any journal in 2009
Number of articles published in that journal in 2007,2008
Number of years – current status = ?state of the art
Rank Abbreviated JournalTitle Impact Factor
1 Cancer Journal for Clinicians 74.575
2 New England Journal of Medicine 50.017
3 Annual Review of Immunology 41.059
4 NATURE REVIEWS MOLECULAR CELL BIOLOGY 35.423
5 PHYSIOLOGICAL REVIEWS 35
6 REVIEWS OF MODERN PHYSICS 33.985
7 Journal of theAmerican Medicel Association 31.718
8 NATURE 31.434
9 CELL 31.253
10 NATURE REVIEWS CANCER 30.762
11 NATUREGENETICS 30.259
12 ANNUAL REVIEW OF BIOCHEMISTRY 30.016
13 NATURE REVIEWS IMMUNOLOGY 30.006
14 NATURE REVIEWS DRUG DISCOVERY 28.69
15 LANCET 28.409
16 SCIENCE 28.103
17 NATURE MEDICINE 27.553
18 ANNUAL REVIEW OF NEUROSCIENCE 26.405
19 NATURE REVIEWS NEUROSCIENCE 25.94
20 ANNUAL REVIEW OFASTRONOMYANDASTROPHYSICS 25.826
Evaluation of quality of references cited
Citation density - average number of
references cited in the new article
Half-life - number of retrospective years
required to find 50% of the cited references
New England Journal of Medicine:
Cited Half-Life: 7.3 years
Immediacy Index is the average number of times an article is cited in the
year it is published.
The journal Immediacy Index indicates how quickly articles in a journal
Immediacy Index = number of citations to articles published in 2008
number of articles published in the journal in 2008
Frequently issued journals have an advantage.
Can identify journals specializing in cutting-edge research.
New England Journal of Medicine
Cites in 2008 to items published in 2008 = 4352
Number of items published in 2008 = 356
Immediacy index = 12.225
Specialty with large number of researchers/practitioners –
more authors, more articles, more citations – numerator
more journals, more articles to cite – denominator
JIF - Basic sciences journals >> Clinical journals
Lowry OH, Rosebrough NJ, Farr AL, et al. Protein measurement with
the folin phenol reagent. J Biol Chem. 1951;193:265-275.
Cited 300,000 times
Southern EM. Detection of specific sequences among DNA fragments
separated by gel-electrophoresis. J Mol Biol. 1975;98:503-517.
Cited 30,000 times
Skewness of citations
The so-called 80/20 phenomenon
20% of articles account for 80% of citations
Citation rates in 1986 or 1987 of
articles published in 3 biochemical
journals in 1983 or 1984,
Seglen, P. O BMJ 1997;314:497
Delay in manuscript acceptance, review and publication = low citation
Many articles on same topic in one issue =increases citation of all
Long-term impact of articles missed – 5-year or 10-year IF
Types of articles: research, review, letters, commentaries,
perspectives, news stories, obituaries, editorials, interviews, and
JAMA published 1905 items - 680 were letters and 253 were
Cited in same year – not included in numerator
Exclude from denominator?
What if cited in numerator in later years?
Selective journal self citation: articles tend to preferentially cite other
articles in the same journal
Self author self citations
Review articles are heavily cited and inflate the impact factor of journals
Long articles collect many citations and give high journal impact factors
Short publication lag permits self citations - high journal impact factor
Journal set in database may vary from year to year
Impact factor is a function of the number of references per article in the
Owned by Garfield / ISI / Thomson Reuters
Includes only journals indexed in “Current Contents”
– another ISI publication – “get indexed with us...”
Coverage of the database is not complete
Database has an English language bias
Database is dominated by American publications
Eigenfactor Project™ is a non-commercial academic research
Sponsored by the Bergstrom lab in the Department of Biology at
the University ofWashington.
Aim to use recent advances in network analysis and information
theory to develop novel methods for evaluating the influence of
scholarly periodicals and for mapping the structure of academic
Committed to sharing findings with interested members of the
public, including librarians, journal editors, publishers, and authors
of scholarly articles.
The Eigenfactor Score measures the number of times articles from
the journal published in the past five years have been cited in the JCR
Like the Impact Factor, the Eigenfactor Score is essentially a ratio of
number of citations to total number of articles. However, unlike the
Impact Factor, the Eigenfactor Score:
Counts citations to journals in both the sciences and social sciences.
Eliminates self-citations. Every reference from one article in a journal
to another article from the same journal is discounted.
Weights each reference according to a stochastic measure of the
amount of time researchers spend reading the journal.
Article Influence Score calculates measures the relative
importance of the journal on a per-article basis.
It is journal's Eigenfactor Score divided by fraction of articles
published by the journal.
That fraction is normalized so that the sum total of articles
from all journals is 1.
The mean Article Influence Score is 1.00.
Score > 1.00 = each article in journal has above-average
Score < 1.00 = each article in the journal has below-average
Hits = readership,
What is the meaning of downloads?
Scholar Google – gives citations
What is the significance?
Cites only from journals?
Citation from websites included?
May not include only peer-reviewed citations