Giving scientific relevance a number.

As much as religious texts are considered direct evidence of the existence of God, scientific publications are the testament of a scientific career. However in the eyes of the scientific community not all papers are created equal and publishing an article can elicit very different reactions depending on the journal in which it is published. These range from the sympathetic nod with feigned knowledge of the journal’s name, to genuine excitement and newfound respect for the author. But in the vast sea of scientific publications, how do people separate the wheat from the chaff? Simply by using one of various types of journal metrics that allow us to know ”how relevant” a published article is.

Impact Factor

The king of kings of journal metrics is Impact Factor (IF). It is the industry standard and it was developed in the 1950s by Eugene Garfield, in an effort to quantify the relative importance of published articles. It is based on two elements:

A: The number of citations in the current year to any items published in the journal in the previous 2 years.

B:The number of “citable items” published in the same 2 years.


For example, if a journal has an impact factor of 5 in 2010, then its papers published in 2008 and 2009 received 5 citations each on average in 2010. However the industry standard is not free from shortcomings, three of the main issues with this metrics system are:

  • High variability across disciplines. Just as music, art or fashion are subject to tremendous differences in popularity between their various styles, science is no exception. One of the most prestigious journals on mathematics, Annals of Mathematics has an IF of 2.928 (2011). In stark contrast the highly regarded journal on biology Cell has an IF of 32.403 (2011).
  • Definition of citable items. Editorial boards on journals tend to be surgically careful in their definition of citable items. In most cases articles in sections titled “letters to the editor” or “brief reports” are not considered to be citable items and therefore do not add to the denominator of the impact factor. However, citations of such items will still contribute to the numerator, thereby inflating the impact factor.
  • Differential effect of individual articles on IF.  In February 2001 the human genome sequence was published in both Science and Nature journals. These articles have accumulated approximately 10000 citations up-to-date, making their contribution to the IF of their respective journal monumental when compared to regular articles.

For some time now, these weaknesses have been a source of concern in the scientific community. After all, why would the most advanced mathematical research be considered less relevant than cutting edge biological studies? In a more fundamental perspective, why should a publication record be vulnerable to changes in journal reputation?

Fortunately in recent years different metrics system have been proposed. Most notably approaches like Source-Normalized Impact per Paper (SNIP), which is based on citations from peer-reviewed articles to other peer-reviewed articles. This prevents abuse of the definition of citable item. SNIP also takes into account citation potential of the scientific field, making comparison between journal subjects possible. Another novel approach is article-level metrics by the Public Library of Science group (PLoS). Along with their open access publishing model, they include article-level metrics, which provide detailed information on article views, number of downloads and citations. Nevertheless these alternative metrics depend on public usage and feedback for their definite acceptance. Only then can there be a change in how we perceive scientific relevance and avoid a standard in which all journals are created equal, but some journals are more equal than others.


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