LETTER TO EDITOR
Year : 2009 | Volume
: 20 | Issue : 2 | Page : 246--248
How useful is journal impact factor?
Department of Epidemiology, College of Public Health and Department of Oral Health Sciences, College of Dentistry, University of Kentucky, Lexington, KY, USA
Department of Epidemiology, College of Public Health and Department of Oral Health Sciences, College of Dentistry, University of Kentucky, Lexington, KY
|How to cite this article:|
Chattopadhyay A. How useful is journal impact factor?.Indian J Dent Res 2009;20:246-248
|How to cite this URL:|
Chattopadhyay A. How useful is journal impact factor?. Indian J Dent Res [serial online] 2009 [cited 2019 Dec 7 ];20:246-248
Available from: http://www.ijdr.in/text.asp?2009/20/2/246/52878
Journal impact factor (JIF) was first described in 1955.  The scientific community recognized early that small but important review journals would not be recognized if calculation of the journal's impact depended solely on the number of publications or citation counts.  Soon, the Institute for Scientific Information (ISI), USA, started using JIF, which is based on two elements: The number of citations in the current year to all items published in a journal in the previous 2 years (the numerator) and the number of substantive articles (source items) published in the same 2 years (the denominator).  If JIF were based on just the preceding year's articles, it would give an even greater weight to rapidly changing fields. However, going beyond 2 years for the source items in the denominator would make JIF less current. 
Librarians, scientists, editors, policy makers, and evaluators use JIF to track the number of citations of a particular article in other works. JIF has often been used for assessing the quality of work of a researcher, faculty, or institution. However, concerns have been raised about such use and about the appropriateness of using JIF as an index of quality of research or scientific productivity. ,, Although some authors have cautioned against use of JIF for assessing quality of research work, very few studies have assessed the JIF of dental journals.,,, How useful is JIF in dentistry? Are articles published in journals with greater JIF necessarily of greater importance?
The denominator used to calculate JIF may have substantial influence on the JIF. If the denominator includes all reports, including those appearing in sections such as 'News,' 'Conference Summaries,' etc., then the JIF would be low as compared to the JIF calculated using only scientific research articles in the denominator. Similarly named section heads in different journals may have contents that are quite different in nature. JIF is also biased towards journals that mature and decline in impact rapidly; it is affected by journal ageing characteristics because it measures the impact of the articles obtained 1 or 2 years after the publication date.
The cited half-life (CHL) is the calculated point (age in year) where 50% of the citations are under the age and 50% of the citations are over the age.  But CHL is an inappropriate measure of decline of journal impact. , Journals that mature slowly but show a rapid decline afterwards (i.e., in the years following the year in which the maximum impact was reached) tend to have higher half-lives (i.e., number of years, going back from the current year, that cover 50% of the citations in the current year to the journal). For example, the half-life of a physiology journal would be longer than that of a journal of molecular biology or astronomy. Recently established journals do not have a 'citable' archive and, consequently, such journals will have higher CHL values than older journals. ,
Although many clinical journals publish high-quality, clinically relevant, and important original studies and systematic reviews, the articles for each discipline are concentrated in a small subset of journals,  the dimensions of which vary according to the particular health care discipline. Overall, across disciplines and study areas, approximately 70% of articles are often found in 30% of journals in any given area of study.  This is consistent with Bradford's Law of Scattering for journal subsets, which states that the important articles on any topic will be concentrated in a small subset of journals, with exponential drop-off in numbers of relevant articles across journal titles.  Citation density (references cited per source article) would be significantly lower for a mathematics article than for a molecular biology article.  Nevertheless, when journals are examined within disciplinary categories, the rankings based on 1-, 7-, or 15-year impact factors do not differ significantly.  Garfield has shown that in each specialty the best journals are those in which it is most difficult to have an article accepted, and these are the journals that have a high impact factor.  These journals and their solid reputations existed long before the impact factor was devised.
An examination of the cited journal list for most titles contains data counting Journal self-citation. One report suggested that self-citation occurred largely at the level of the individual title and removal of self-citations from JIF calculation had little effect on the relative rank of high-impact journals. Although some journals with lower JIF and lower rank in category show more dependence on the contribution of self-citations, only a small proportion of journals show significant change in quartile rank following the removal of self-citations. 
Another factor is that several major journals require authors to keep references limited to a specified period, number, or certain types of articles. Differential citation patterns may be generated due to journal policy. For example, the Journal of Clinical Periodontology states: 'It is the policy of the Journal to encourage reference to the original papers rather than to literature reviews.' In contrast, the Journal of Periodontal Research is more relaxed, stating that references be kept 'to a pertinent minimum.' Variations in the number of references allowed by a journal severely bias the JIF. An example of this variation can be seen in the case of Oral Oncology, which sets a limit of 30 references for original papers and 10 for short communications. In such cases, differential citation patterns may develop if short communications and original papers are provided equal weight in calculating JIF.
Publication frequencies of journals also vary: The JIF is biased against a journal that has a lower frequency of publication. Although certain journals may have high reputations and impact within a discipline/ specialty/ subspecialty, their JIF may be low because of fewer numbers of publications per year. High impact factor journals may have a larger readership than low impact factor journals which are usually specialty journal, and journals targeting a particular type of research or readership. A smaller group of professionals will cite articles from these latter journals, thereby feeding into a vicious cycle and sustaining the low JIF.
The ever-increasing number of new journals has a major impact on the redistribution of articles. Authors frequently face the need for quick publication, while the publication process is time consuming. This conflict between the need for early publication and the long review time required for journals can result in articles being routed towards journals with shorter turn-around time, independent of JIF; this is seen in the increase in popularity of the new online publications.
Journals advertise their JIF as a marketing tool to attract authors to submit manuscripts. JIF is known to have more immediacy value and little archival value, which begs the question: Should published articles have a 'potent period' beyond which their effects should be considered as having 'expired'? Articles aggregate and cluster in journals. Assessment of the merit of a scientific work or a scientist, or the cumulative merit over a period of time, requires a holistic approach; it should take into consideration, for example, the pool of journals of merit where the work has been published and not just the JIF. JIF should be only one constituent of a set of criteria for judging the merit of a published work. JIF may not be a true marker of the contextual importance of dental journals and of the value of the articles published in them.
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