Cited by 828scientometrics and bibliometrics researchWeb visibility and informetricsPsychology Research and Bibliometrics

Universality of citation distributions: Toward an objective measure of scientific impact

Open access
Radicchi, Filippo, Fortunato, Santo, Castellano, Claudio·2008·Proceedings of the National Academy of Sciences

Abstract

We study the distributions of citations received by a single publication within several disciplines, spanning broad areas of science. We show that the probability that an article is cited c times has large variations between different disciplines, but all distributions are rescaled on a universal curve when the relative indicator c f = c / c 0 is considered, where c 0 is the average number of citations per article for the discipline. In addition we show that the same universal behavior occurs when citation distributions of articles published in the same field, but in different years, are compared. These findings provide a strong validation of c f as an unbiased indicator for citation performance across disciplines and years. Based on this indicator, we introduce a generalization of the h index suitable for comparing scientists working in different fields.

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Written from the paper’s full text.

TL;DR

The paper shows that citation distributions across scientific disciplines and publication years collapse onto a universal curve when citations are rescaled by the field-year average \(c_0\), making \(c_f=c/c_0\) an unbiased relative indicator. It introduces a generalized h-index \(h_f\) based on \(c_f\) and normalized publication rates.

Key findings

  • Bare citation distributions vary strongly by discipline: a publication with 100 citations is approximately 50 times more common in Developmental Biology than in Engineering, Aerospace. After rescaling by \(c_0\), all distributions collapse onto a universal curve fitted by a lognormal with \(\sigma^2=1.3\).
  • Universality also holds across years within the same field: for Hematology, Neuroimaging, and Physics, Nuclear, articles from 1990, 1999, and 2004 show the same rescaled distribution despite \(c_0\) growing with elapsed time.
  • Ranking articles by \(c_f\) reduces field bias: for the top \(z=20\%\), the standard deviation of discipline bin heights is \(\sigma_z=1.15\%\) for \(c_f\)-based ranking versus \(12.37\%\) for bare \(c\), compared with the theoretical unbiased value \(\sigma_z=1.09\%\).
  • The generalized h-index \(h_f\) orders articles by \(c_f\) and uses reduced rank \(r/N_0\); an example with \(c_f\) values 4.1, 2.8, 2.2, 1.6, 0.8, 0.4 and \(N_0=2.0\) gives \(h_f=1.5\). Author publication counts also rescale, fitted by a power law with exponent \(-3.5(5)\).

Methods

  • Data come from Thomson Scientific’s Web of Science; journals are divided into 172 JCR categories, and only records classified as “article” and “letter” are considered. Citations are counted as the total number of times an article appears as a reference in a more recent published article.
  • For each discipline and year, the average citations per article \(c_0\) is computed; distributions \(P(c,c_0)\) are rescaled as \(c_0P(c,c_0)\) versus \(c_f=c/c_0\). Curves are fitted with a lognormal constrained by \(\sigma^2=-2\mu\) because the expected value of \(c_f\) is 1, using exponentially growing bins.
  • Ranking analysis pools disciplines, ranks articles by \(c\) and by \(c_f\), and compares the percentage of each discipline in the top \(z\%\) against an unbiased-ranking expectation. The generalized h-index is defined using \(c_f\) and \(r/N_0\).

Limitations

  • Stated: uncited articles are neglected, though the authors verified their inclusion only produces a small shift in \(c_0\); “Multidisciplinary sciences” does not fit perfectly because its citation distribution is a convolution of single disciplines, but restricting to Nature, Science, and PNAS fits well.
  • Stated: the analysis addresses single publications, not individuals or research groups, and other biases such as number of authors are left for future work; the origin of the universality is also not understood.
  • Evident from the text: JCR categories are not mutually exclusive, and the generalized h-index’s practical application is deferred to a forthcoming publication.

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Integrity signals

Registry

Crossref · 10.1073/pnas.0806977105

Cites

35 of 35 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 7 cited sources checked · 0 supported · 0 flagged

  • Findingown data
    § II. VARIABILITY OF CITATION STATISTICS IN DIFFERENT DISCIPLINES · p. 2

    A publication with 100 citations is approximately 50 times more common in Developmental Biology than in Engineering, Aerospace.

    “For example a publication with 100 citations is approximately 50 times more common in 'Developmental Biology' than in 'Engineering, Aerospace'.”
    Quote in the paperSubstantial claim
  • Findingown data
    § III. DISTRIBUTION OF THE RELATIVE INDICATOR c f · p. 2

    The universal rescaled citation distribution resembles a lognormal distribution with σ^2 = 1.3 when a single curve is fitted for all categories.

    “Fitting a single curve for all categories, a lognormal distribution with σ 2 = 1 . 3 is found, that is reported in Figure 2.”
    • Also stated in: § II. VARIABILITY OF CITATION STATISTICS IN DIFFERENT DISCIPLINES · p. 2 · plain
      “The dashed line is a lognormal fit with σ 2 = 1 . 3.”
    Quote in the paperSubstantial claim
  • Findingown data
    § III. DISTRIBUTION OF THE RELATIVE INDICATOR c f · p. 2

    Values of χ2 per degree of freedom indicate that the lognormal fit is good.

    “Values of χ 2 per degree of freedom, also reported in Table I, indicate that the fit is good.”
    Quote in the paperSubstantial claim
  • Findingown data
    § Universality of citation distributions: towards an objective measure of scientific impact. · p. 1

    The same universal behavior occurs when citation distributions of articles published in the same field but in different years are compared.

    “In addition we show that the same universal behavior occurs when citation distributions of articles published in the same field, but in different years, are compared.”
    • Also stated in: § III. DISTRIBUTION OF THE RELATIVE INDICATOR c f · p. 4 · plain
      “The value of c 0 obviously grows as older publications are considered, but the rescaled distribution remains conspicuously the same.”
    • Also stated in: § IV. A GENERALIZED H-INDEX · p. 5 · plain
      “the universal scaling observed over different disciplines naturally holds also for articles published in different periods of time.”
    Quote in the paperSubstantial claim
  • Findingown data
    § IV. A GENERALIZED H-INDEX · p. 4

    The average number of articles published by an author in a year depends on the discipline.

    “As Fig. 5 shows, such a quantity also depends on the discipline considered: in some disciplines the average number of articles published by an author in a year is much larger than in others.”
    Quote in the paperSubstantial claim
  • May combine two claims. 5
  • Classified as probably not a substantial claim. 1
  • May combine two claims.

    Findingown data
    § III. DISTRIBUTION OF THE RELATIVE INDICATOR c f · p. 3

    For z = 20%, the standard deviation of bin heights is 1.15% for c_f-based ranking and 12.37% for c-based ranking, compared with 1.09% expected under unbiased ranking.

    “For example, for z = 20%, σ z = 1 . 15% for c f -based ranking, while σ z = 12 . 37% if c is used, as opposed to the value σ z = 1 . 09% in the hypothesis of unbiased ranking.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Under an unbiased ranking, articles of the various disciplines are scattered uniformly along the rank axis.
  • May combine two claims.

    Findingown data
    § III. DISTRIBUTION OF THE RELATIVE INDICATOR c f · p. 2

    All fitted values of σ^2 for the disciplines are compatible within two standard deviations, except Anesthesiology, which is within three standard deviations of all the others.

    “All fitted values of σ 2 , reported in Table I, are compatible within two standard deviations, except for one (Anesthesiology) that is in any case within three standard deviations of all the others.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data
    § III. DISTRIBUTION OF THE RELATIVE INDICATOR c f · p. 3

    When articles from different disciplines are ranked by c_f, the percentage of each discipline in the top z% agrees well with the unbiased-ranking expectation, whereas ranking by bare c shows strong bias.

    “When the ranking is performed according to c f = c/c 0 we find (Table II) a very good agreement with the hypothesis that the ranking is unbiased, while strong evidence that the ranking is biased is found when c is used.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Under an unbiased ranking, articles of the various disciplines are scattered uniformly along the rank axis.
  • May combine two claims.

    Findingown data
    § III. DISTRIBUTION OF THE RELATIVE INDICATOR c f · p. 4

    The value of c0 grows as older publications are considered, but the rescaled citation distribution remains the same.

    “The value of c 0 obviously grows as older publications are considered, but the rescaled distribution remains conspicuously the same.”
    Quote in the paperSubstantial claim
  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation
    § III. DISTRIBUTION OF THE RELATIVE INDICATOR c f · p. 4

    Different temporal patterns of citations exist, with some articles starting soon to receive citations while others ('sleeping beauties') go unnoticed for a long time before attracting many citations.

    “It is known that different temporal patterns of citations exist, with some articles starting soon to receive citations, while others ('sleeping beauties') go unnoticed for a long time, after which they are recognized as seminal and begin to attract a large number of citations [26, 27].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [26] Van Raan AF (2004) Sleeping Beauties in Science. Scientometrics 59:461-466. source

      Not checked in the free run.

    • cites [27] Redner S (2005) Citation statistics from 110 years of Physical Review. Phys Today 58:49-54. source

      Not checked in the free run.

5 of 5
  • The authors state this as an assumption.

    Under an unbiased ranking, articles of the various disciplines are scattered uniformly along the rank axis.

    “assuming that articles of the various disciplines are scattered uniformly along the rank axis, one would expect the average bin height in Fig. 3 to be z % with a standard deviation”
1 of 1

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • The number of citations a paper receives is the most commonly used measure of scientific impact.

    “The number of citations a paper receives is the most commonly used measure of scienti fi c impact.”

    From Unraveling scientific impact: Citation types in marketing journals (2015) · cites this paper as Radicchi, Fortunato, & Castellano, 2008

  • Variability in citation practices across disciplines prevents direct comparability of citation counts across fields unless appropriate corrective methods are applied.

    “Variability in citation practices across disciplines, for instance, is a well-recognized problem which prevents direct comparability of citation counts across fi elds (Bornmann & Daniel, 2008), unless appropriate corrective methods are applied (Radicchi et al., 2008).”

    From Unraveling scientific impact: Citation types in marketing journals (2015) · cites this paper as Radicchi et al., 2008

  • Recent work has improved theoretical understanding of citation dynamics.

    “Recent work has improved our theoretical understanding of citation dynamics [27–29].”

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Authors
Radicchi, F., Fortunato, S., & Castellano, C.
Affiliations
  • Institute for Scientific Interchange (IT)
  • National Research Council (IT)
Corresponding author
Castellano, Claudio
Venue
Proceedings of the National Academy of Sciences
Published
2008-11-11
Volume
105
Issue
45
Pages
17268-17272
ISSN
  • 0027-8424 (ISSN-L)
  • 1091-6490
OpenAlex
W2151866568
Cited by
828
References
31
Citations by year
2026: 20 · 2025: 38 · 2024: 42 · 2023: 40 · 2022: 41 · 2021: 48 · 2020: 53 · 2019: 64 · 2018: 44 · 2017: 75 · 2016: 58 · 2015: 56 · 2014: 57 · 2013: 61 · 2012: 48
FWCI
31.18
Type
journal article
Language
en
Open access
Locations
  • Proceedings of the National Academy of Sciences (publishedVersion)
  • PubMed Central (publishedVersion)
  • PubMed (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • arXiv (Cornell University)
Topics
  • Social Sciences › Decision Sciences › Statistics, Probability and Uncertainty › scientometrics and bibliometrics research
  • Physical Sciences › Computer Science › Information Systems › Web visibility and informetrics
  • Social Sciences › Psychology › Experimental and Cognitive Psychology › Psychology Research and Bibliometrics
Keywords
citation distribution, scientific impact, h-index, cross-disciplinary comparison, rescaling, citation performance, bibliometric analysis
MeSH terms
Models, Theoretical, Selection Bias, Interdisciplinary Communication, Journal Impact Factor

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