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Citation in the Electronic Age

December 15, 2014

Science is an example of the "standing on the shoulders of giants" metaphor. Starting from their textbooks, and continuing through the many journal articles that they read, scientists build on what has been discovered before them. Isaac Newton used this metaphor in a February 5, 1676, letter to Robert Hooke, and it's imprinted on the British two pound coin.[1] Scientists are more neutral subjects for currency than politicians.[2]

Scientists cite previous work in their publications, but they need to limit the number of these citations to those few that they think are especially relevant. Obviously, all planetary studies owe a deep debt of gratitude to Copernicus, but planetary scientists generally do not cite his "De revolutionibus orbium coelestium."

Nicolaus CopernicusPortrait of Nicolaus Copernicus

Copernicus also had some shoulders to stand on. Heliocentrism was first proposed by Aristarchus of Samos (c. 250 BC).

(From Porträts berühmter Persönlichkeiten (Strassburg, 1587), by Nicolaus Reusner, via Wikimedia Commons.)

For the same "standing on shoulders" reason, recent research papers have more relevance as resources, so they're cited often after they're first published, but then fade into obscurity. As I wrote in a previous article (Modeling Scientific Citation, December 16, 2011), physicists from the University of Fribourg (Fribourg, Switzerland) found that the frequency of new citations that a paper receives declines dramatically after just a few years.[3-4] Examining the citations of 450,000 papers published from 1893 to 2009 in American Physical Society journals, they found that the citation frequency appears to be an exponential decay at longer times.[4]

Citation distribution of APS journal papersTime decay of paper relevance, which is a function of its citations in 91-day intervals, as a function of time. The data are grouped into three sets, as defined in ref. 4. This is a simplified version of fig. 1 of ref. 4.

(Via arXiv).[4]

One of the more interesting things discovered by the Swiss physicists is the revelation that 60,000 of the 450,000 papers (13%) weren't cited at all, at least in APS journals in their dataset.[3-4] It appears that many scientists are like the vast majority of novelists, who devote many hundreds of man-hours to their work, carefully wording and proofing a publication that no one reads. In this respect, The Two Cultures are very similar, at least at the lower ranks.

Now that journal articles have become available online, has this new form of "reference library" influenced scientific citation? A 2008 study published in the Journal of the American Society for Information Science and Technology found that the age of cited articles has increased continuously since the mid-1960s; that is, researchers were referring to an increasingly older body of literature.[5] It was found, also, that the easy availability of recent articles on arXiv had the opposite effect in some physics subfields. Scientists were just looking at what came easily to their desktops.[5]

A recent study on this topic, posted on arXiv, confirms the finding of overall citation aging found in the 2008 study.[6-7] The seven member research team from Google, Inc., used Google Scholar tools[8-9] to study citations from articles published in the period 1990-2013. In their study, they defined an "older article" as being one that was published at least 10, 15, or 20 years before the citing article, and they calculated the fraction of older citations for these ranges.

The Google team found the following:

• Citation of older articles has grown substantially from 1990 to 2013. In 2013, 36% of citations were for articles at least 10 years old, up from 28% in 1990.[6]

• The increase over the second half of the period (2002-2013) was twice the increase for the first half (1990-2001).[6]

• This trend exists for older articles, also. In 2013, 21% of citations were to articles 15 years old and older, an increase of 30% over the 1990 level. Thirteen percent of citations were to articles 20 years old or older, an increase of 36% over the 1990 level.[6]

A graphical summary of one important result is shown in the figure.

Age trend in research paper citations
Fraction of citations to older articles in published research, where "older" is defined as either fifteen or twenty years. (Fig. 3 of ref. 6, redrawn for clarity, via arXiv.)[6]

The Google team ascribes the cause of this trend to the now easy way that relevant older articles can be accessed.[6] I was fortunate in having spent my career at places with large university and corporate libraries, but that advantage is not held by all scientists. In this case, the Internet is the great equalizer.


  1. Two Pound Coin Designs and Specifications, The Royal Mint Limited, 2014.
  2. Steven A. Feller, "20th Century Physicists On Bank Notes," Radiations (Magazine of the Sigma Pi Sigma Physics Honor Society), Fall 2010 (0.5 MB PDF File).
  3. Matúê Medo, Giulio Cimini, and Stanislao Gualdi, "Temporal Effects in the Growth of Networks," Physical Review Letters, vol. 107, no. 23 (December 2, 2011), Document No. 238701 (4 pages).
  4. Matúê Medo, Giulio Cimini, and Stanislao Gualdi, "Temporal Effects in the Growth of Networks," arXiv Preprint Server, September 26, 2011.
  5. Vincent Larivière, Éric Archambault, and Yves Gingras, "Long-term variations in the aging of scientific literature: From exponential growth to steady-state science (1900–2004)," Journal of the American Society for Information Science and Technology, vol. 59, no. 2 (January 15, 2008), pp. 288-296.
  6. Alex Verstak, Anurag Acharya, Helder Suzuki, Sean Henderson, Mikhail Iakhiaev, Cliff Chiung Yu Lin, and Namit Shetty, "On the Shoulders of Giants: The Growing Impact of Older Articles," arXiv, November 2, 2014.
  7. John Bohannon, "Older papers are increasingly remembered—and cited," Science, November 4, 2014.
  8. Google Scholar Citations
  9. Google Scholar Metrics

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