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G Currie, J Wheat. Trends analysis of impact factors for nuclear medicine journals. The Internet Journal of Nuclear Medicine. 2007 Volume 5 Number 1.
Despite debate as to its usefulness, the journal impact factor remains the key statistic gleaned from the journal citation report. An analysis of trends in nuclear medicine impact factors is yet to be reported in the literature. Moreover, the relationship between impacts factors and the other indices contained in the journal citation report are complex and seldom considered in interpreting the implication of impact factors in decision making.
The 2006 journal citation report showed that nuclear medicine journals have demonstrated a trend towards increased impact factors. The
Thomson Scientific, or the Institute of Scientific Information (ISI), publishes in June each year the Journal Citation Reports (JCR). Despite debate as to its usefulness, the journal impact factor (IF) remains the key statistic gleaned from the report. The role and limitations of impact factors have been previously described (
The Impact Factor
A journals' impact factor is a measure of the ratio of recent citations to recent articles in a particular journal. The impact factor is the number of times the ‘average' article in a journal over the preceding two years was cited in journals in the subsequent year. That is, the 2006 impact factor is based on the number of citations in all ISI listed journals in 2006 of articles published in the specific journal in 2005 and 2004.
The ISI JCR offers 172 various categories for journals ranging from ‘acoustics' to ‘zoology'. Nuclear medicine journals generally fit the ‘radiology, nuclear medicine and medical imaging' category, however, a number of nuclear medicine journals may be found in other categories. For example,
Within the ‘radiology, nuclear medicine and medical imaging' category in 2006 there were 85 journals of which just 11 are specifically nuclear medicine. The impact factor rating of nuclear medicine journals since 1998 has been summarized in table 1. A number of trends are evident:
The J Nucl Med has consistently ranked highest amongst nuclear medicine journals.
The impact factor for nuclear medicine journals has steadily increased consistent with the trend for the broader ISI category (figure 1).
A sharp upward spike in impact factor was generally seen amongst nuclear medicine journals in 2004. Most notably, Clin Nucl Med soured by 114.7% and Semin Nucl Med jumped 69.0%.
Those with a prominent 2004 spike tended to suffer a marked decrease in impact factor in 2005, generally below the figure of 2003. Decreases below 2003 scores were seen for
The German language journal,
In 2006, the
The 2004 general increase in impact factor followed by the subsequent 2005 decrease may represent penetration of innovation in nuclear medicine in other disciplines. That is, 2004 may represent a burst of referencing to nuclear medicine journals outside the discipline for new technology and techniques. An example might include the emergence of PET/CT or SPECT/CT articles in radiology journals and/or oncology journals. An inspection of the relatedness indices for 2003 and 2004 for
One of the obvious limitations of the impact factor is that it provides a defined 2 year period for calculations. The 2 year window is relatively arbitrary and perhaps would be better suited or be more equitable if a rolling 2 year window were employed. That is, the impact factor is determined for 2006 publications for citations to the years 2004 and 2005. Clearly a December 2006 article could cite a January 2004 article giving an effective currency window of virtually 3 years. This would be particularly advantageous to those journals with 12 or more editions annually.
More importantly, perhaps, is cycle time. Leading journals in respective discipline areas may be cited within the same year of publication. This perhaps provides an indicator of high impact and/or quality yet is excluded from the impact factor calculation. It may also be a function of the average peer review period, the average acceptance to publication period and/or the liquidity of innovation and development in the given discipline. The immediacy index attempts to capture rapid citation. A journals impact or quality might also be highlighted by prolonged citation of articles beyond the 2 year sampling window. The cited half life and citing half life attempt to capture this omission from the impact factor.
The Immediacy Index
The immediacy index (II) is a measure of the citation rate to current articles. The immediacy index is the number of times the ‘average' current article in a journal was cited in journals in the current year. That is, the 2006 immediacy index is based on the number of citations in all ISI listed journals in 2006 of articles published in the specific journal in 2006. Table 2 provides a summary of the immediacy indices for nuclear medicine journals in 2005 and 2006
The immediacy index should be interpreted as an adjunct to impact factor assessment. Figures 2 and 3, however, indicates that for both nuclear medicine journals and all journals included in the ISI JCR, the impact factor and immediacy index are strongly correlated. Thus, the impact factor also provides a reflection of immediacy index.
Cited and Citing Half Lives
The cited half life is a measure of the median age of a journals cited articles in the current year. That is, half of the citations to articles of a journal in the current year will have been published within the cited half life. Table 3 provides a summary of the cited half lives for nuclear medicine journals in 2005 and 2006.
Cited half life (2006) = years for most recent 50% of citations received by a journal in all 2006 articles
The citing half life is a measure of the median age of an article a journal cited in the current year. That is, half of the cited articles included in a journal in the current year will have been published within the citing half life. Table 3 provides a summary of the citing half lives for nuclear medicine journals in 2005 and 2006
Citing half life (2006) = years for most recent 50% of all citations in 2006 articles of a journal
The cited and citing half lives are a simple calculation with complex implications. One perspective might be that a long cited half live indicates that the impact factor underestimates the longevity of published work in that field. The high cited half life for the
The citing half life might provide a more useful tool for measuring journal quality. A long citing half life indicates that the articles in a journal in any given year predominantly use references to journal articles published some years earlier. Again, this may simply reflect a more sustained period of relevance of published work in that field. However, a negative correlation was noted between a journals impact factor and the citing half life (figure 5). Presumably, this reflects higher impact journals focusing on more ‘current' issues within the profession and, thus, citing more recent articles.
The JCR provides a breakdown for each journal of which journals cite articles from that journal (cited) and which journals are cited by that journal (citing). A journal relatedness index reflects the relationship between cited and citing behaviour of each journal. Not surprisingly, nuclear medicine journals generally show strong relatedness to other nuclear medicine journals. There is, however, variations in the strength of relationships that are not elucidated by the cited maximum relatedness index. For example,
Journal visibility might be highlighted by the number of journals contained in the relatedness index report. A greater number of journals included in the report would indicate a broader influence. Table 5 provides a breakdown of the number of journals included in the relatedness index for nuclear medicine journals. Not surprisingly, there was a general correlation between journal number and the impact factor. Lack of visibility was generally associated with specialized journal factors (review article, non English language or sub specialty focus).
Number and Type of Articles
The impact factor makes no account of the number of articles a given journal contributes to industry. Indeed, high impact factors are often associated with journals publishing less than 20 articles over the 2 year sampling period. While the category of ‘radiology, nuclear medicine and medical imaging' includes a three journals offering less than 20 articles in the 2004 and 2005 period (
Self citation is the practice of referencing ones own previously published work in a manuscript (
The Quality Factor
Ironically, the impact factor does not, in fact, attempt to measure a journals impact but rather its quality. While the impact factor provides a useful rudimentary tool for comparing journal quality within a discipline area, it needs to be interpreted in conjunction with the full JCR. Still, the tool is imperfect in differentiating quality and impact amongst related journals. Moreover, the numerical differences in impact factors offers little insight into the relative impact or quality margin. There is, however, a tendency amongst users to rely on a single index of measure.
Nuclear medicine journals have demonstrated a trend towards increased impact factors in the 2006 JCR. Moreover, nuclear medicine journals have shown an aggregated impact factor increasing at a more rapid rate than that of the ‘radiology, nuclear medicine and medical imaging' category. The
Geoff Currie School of Dentistry and Health Sciences Locked Bag 588 Charles Sturt University Wagga Wagga 2678 Australia Telephone: 61 2 69332822 Facsimile: 61 2 69332587 Email: email@example.com