Journal Impact Factors: What Are They

& How Can They Be Used?

Pamela Sherwill, MLS, AHIP

April 27, 2004

History Of Impact Factors

• Created in the 60s to select journals for Science Citation Index

• Developed to compare journals regardless of their size

• Journal Impact Factors (IF) = Article IF• Size & breadth of a scientific field

determines “super-cited” papers • Delays in reviewing and publication affects

IF

Impact Factor:A Definition

Journals with high IF publish articles that are cited more often than journals with lower IF. If citation numbers are taken as a measure of quality, then these journals are high ranking by this measure.

Key Determinants

The key determinants are not the number of authors or articles in the field but the citation density and the age of the literature cited. The average number of citations per article and the immediacy of citation are the significant elements. The size of a field will generally increase the number of ‘super-cited’ papers.

---Eugene Garfield

What Is An Impact Factor ?

• # of current citations a journal receives divided by the number of articles published in the two preceding years

Citation Half Life• How long articles in a journal continue

to be cited after publication

Citation Density

• Mean # of references cited per article • Varies by discipline – lower in math than life

sciences• Higher in review articles

Half-Life

• # of retrospective years required to find 50% of the cited references

Calculating Impact Factors

# of citations in the current year for a journal

# items published in the journal for the last 2 years

What Influences IF?

• Review articles cited more often

• Case reports rarely cited

• Rapid publication time >

• Self-citations >

• Bias towards rapidly evolving fields

• Cites not counted after 2 years

• Specialty journals have < IF

Impact Factors

• Scientific journals score > than clinical ones

• US journals score > than European

• Review articles score > than original articles

• Methodological papers may score > than those with new data

• Free electronic access > the IF of a journal

How Are IFs Used?

• Judge a publication’s quality or prestige

• Assess academic productivity

• Authors choosing where to publish

• Evaluate an author or journal editor

• Decisions for tenure & promotion

• By libraries to make collection decisions

Nursing Journal Facts

• Journal Citation Reports (the source of IFs) rates 33 general nursing journals

• PubMed indexes 248 refereed nursing journals

• CINAHL indexes 548 active nursing journals

• ISI regards librarians as their primary customers

• ISI to > # of Nursing journals by 23 in 2006

Determining Journal Quality

• Nursing limited # of journals rated (33)

• General nursing journals – not specialty

• Nursing journals – impact factors <2

• IF does not measure impact of specific articles

• Not every article in a high impact journal is of high quality

Self-Citations

• Encouraged by some editors/journals

• Not subtracted when IF is calculated

• Authors accumulate many self-citations

• ISI claims it has little effect on the relative rank of highly ranked journals

• Journals with IF < .5 have high self- citation rates

Issues With JCR Database

• Prestige/quality is a murky concept

• Recent articles not enough time to be cited

• Citations not evenly distributed among articles in an issue

• Journal impact factor not article impact factor

• Pure clinicians read clinical articles but do not write or cite

Issues With JCR Database(continued)

• Rapidly expanding fields tend to have >IF

• Letters, editorials, and news items not counted in article total but if cited are counted as citations for the journal

• Small # of articles lead to a large proportion of citations in a journal

• Limited # of evaluated journals

Impact Factor Limitations

• Reflect the journal rather than the article• Vary with time in numbers & ranking• Changes in clinical interest affects IF• Not related to the peer review process• Can be manipulated by authors or editors• 2-year period is arbitrary - not empirically based• Journal availability affects the ranking• Author citation errors

Limitations (continued)

• Journal IF involve large populations of articles and citations

• Authors produce smaller numbers of articles• 80/20 rule – 20% of the articles account for 80%

of the citations• IF can vary from journal issue to issue• IF vary from year to year• Lack of empiric studies on IF as measure of

quality

Impact Factor Alternatives

• No other formal evaluation tool

• Professional recommendations

• Refereed journals

• Editor’s reputation

• Editorial standards

• Experience & stature of editor & board

Alternatives

• Time from acceptance to publication

• Acceptance/rejection rate

• Best quality journals are often most competitive in acceptance for publication

Impact factor for different journals—recent.

Name of journal Impact factor

New Eng J Med 50

Lancet 28.4

JAMA 28.8

BMJ 12.8

Older..

Thank You