Citation Advantage For OA Self-Archiving Is Independent of Journal Impact Factor, Article Age, and Number of Co-Authors

From: Stevan Harnad <>
Date: Wed, 17 Jan 2007 19:24:19 +0000

    Full text, with figure and hyperlinks, is at:

SUMMARY: Eysenbach has suggested that the OA (Green) self-archiving
advantage might just be an artifact of potential uncontrolled
confounding factors such as article age (older articles may be both
more cited and more likely to be self-archived), number of authors
(articles with more authors might be more cited and more
self-archived), subject matter (the subjects that are cited more,
self-archive more), country (same thing), number of authors, citation
counts of authors, etc.
      Chawki Hajjem (doctoral candidate, UQaM) had already shown that
the OA advantage was present in all cases when articles were analysed
separately by age, subject matter or country. He has now done a
multiple regression analysis jointly testing (1) article age, (2)
journal impact factor, (3) number of authors, and (4) OA
self-archiving as separate factors for 442,750 articles in 576
(biomedical) journals across 11 years, and has shown that each of the
four factors contributes an independent, statistically significant
increment to the citation counts. The OA-self-archiving advantage
remains a robust, independent factor.
      Having successfully responded to his challenge, we now challenge
Eysenbach to demonstrate -- by testing a sufficiently broad and
representative sample of journals at all levels of the journal
quality, visibility and prestige hierarchy -- that his finding of a
citation advantage for Gold OA (articles published OA on the
high-profile website of the only journal he tested (PNAS) over Green
OA articles in the same journal (self-archived on the author's
website) was not just an artifact of having tested only one very
high-profile journal.
In May 2006, Eysenbach published "Citation Advantage of Open Access
Articles" in PLoS Biology, confirming -- by comparing OA vs. non-OA
articles within one hybrid OA/non-OA journal -- the "OA Advantage"
(higher citations for OA articles than for non-OA articles) that had
previously been demonstrated by comparing OA (self-archived) vs.
non-OA articles within non-OA journals.

This new PLoS study was based on a sample of 1492 articles (212 OA,
1280 non-OA) published June-December 2004 in one very high-impact
(i.e., high average citation rate) journal: Proceedings of the
National Academy of Sciences (PNAS). The findings were useful because
not only did they confirm the OA citation advantage, already
demonstrated across millions of articles, thousands of journals, and
over a dozen subject areas, but they showed that that advantage is
already detectable as early as 4 months after publication.

The PLoS study also controlled for a large number of variables that
could have contributed to a false OA advantage (for example, if more
of the authors that chose to provide OA had happened to be in subject
areas that happened to have higher citation counts). Eysenbach's
logistic and multiple regression analyses confirmed that this was not
the case for any of the potentially confounding variables tested,
including the (i) country, (ii) publication count and (iii) citation
count of the author and the (iv) subject area and (v) number of
co-authors of the article.

However, both the Eysenbach article and the accompanying PLoS
editorial, considerably overstated the significance of all the
controls that were done, suggesting that (1) the pre-existing
evidence, based mainly on OA self-archiving ("green OA") rather than
OA publishing ("gold OA"), had not been "solid" but "limited" because
it had not controlled for these potential "confounding effects." They
also suggested that (2) the PLoS study's finding that gold OA
generated more citations than green OA in PNAS pertained to OA in
general rather than just to high-profile journals like PNAS (and that
perhaps green OA is not even OA!):
Eysenbach (2006): "[T[he [prior] evidence on the ?OA advantage? is
controversial. Previous research has based claims of an OA citation
advantage mainly on studies looking at the impact of self-archived
articles... (which some have argued to be different from open access
in the narrower sense)... All these previous studies are
cross-sectional and are subject to numerous limitations... Limited or
no evidence is available on the citation impact of articles originally
published as OA that are not confounded by the various biases and
additional advantages [?] of self-archiving or ?being online? that
contribute to the previously observed OA effects."

PLoS Editorial (MacCallum & Parthasarathy 2006): "We have long argued
that papers freely available in a journal will be more often read and
cited than those behind a subscription barrier. However, solid
evidence to support or refute such a claim has been surprisingly hard
to find. Since most open-access journals are new, comparisons of the
effects of open access with established subscription-based journals
are easily confounded by age and reputation... As far as we are aware,
no other study has compared OA and non-OA articles from the same
journal and controlled for so many potentially confounding factors...
The results... are clear: in the 4 to 16 months following publication,
OA articles gained a significant citation advantage over non-OA
articles during the same period... [Eysenbach's] analysis [also]
revealed that self-archived articles are... cited less often than OA
[sic] articles from the same journal."
When I pointed out in a reply that subject areas, countries and years
had all been analyzed separately in prior within-journal comparisons
based on far larger samples, always with the same outcome -- the OA
citation advantage -- making it highly unlikely that any of the other
potentially confounding factors singled out in the PLoS/PNAS study
would change that consistent pattern, Eysenbach responded:
Eysenbach: "[T]o answer Harnad's question 'What confounding effects
does Eysenbach expect from controlling for number of authors in a
sample of over a million articles across a dozen disciplines and a
dozen years all showing the very same, sizeable OA advantage? Does he
seriously think that partialling out the variance in the number of
authors would make a dent in that huge, consistent effect?' ? the
answer is ?absolutely?.
My doctoral student, Chawki Hajjem, has accordingly accepted
Eysenbach's challenge, and done the requisite multiple regression
analyses, testing not only (3) number of authors, but (1) number of
years since publication, and (2) journal impact factor. The outcome is
that (4) the OA self-archiving advantage (green OA) continues to be
present as a robust, independent, statistically significant factor,
alongside factors (1)-(3):
(1) number of years since publication (BLUE)
(2) journal impact factor (additional variable not tested by
Eysenbach) (PURPLE)
(3) number of authors (RED)
(4) OA self-archiving (GREEN)

Already tested separately and confirmed:
(5) country (previously tested: OAA separately confirmed for all
countries tested -- 1st author affiliation)
(6) subject area (previously tested: OAA separately confirmed in all
subject areas tested)

Not tested:
(7) publication and citation counts for first and last authors (not
tested, but see Moed 2006)

(8) article type (only relevant to PNAS sample)
(9) submission track (only relevant to PNAS sample)
(10) funding type (irrelevant)

Independent effects of (1) Year of Publication (purple), (2) Journal
Impact Factor (blue), (3) Number of Authors (red) and (4) OA
Self-Archiving (green) on citation counts: Beta weights derived from
multiple regression analyses of (column 1) raw distribution, (column
2) log normalized distribution, (columns 3-6) separate Journal Impact
Factor Quartiles, and (columns 7-10) separate Year of Publication
Quartiles. In every case, OA Self-Archiving makes an independent,
statistically significant contribution (highest for the most highly
cited articles, column 6 "Groupe Dri": i.e., the QA/QB effect).
(Biology, 1992-2003; 576 journals; 442,750 articles). For more details
see Chawki Hajjem's website.
In order of size of contribution:

Article age (1) is of course the biggest factor: Articles' total
citation counts grow as time goes by.

Journal impact factor (2) is next: Articles in high-citation journals
have higher citation counts: This is not just a circular effect of the
fact that journal citation counts are just average journal-article
citation counts: It is a true QB selection effect (nothing to do with
OA!), namely, the higher quality articles tend to be submitted to and
selected by the higher quality journals!.

The next contributor to citation counts is the number of authors (3):
This could be because there are more self-citations when there are
more authors; or it could indicate that multi-authored articles tend
to be of higher quality.

But last, we have the contribution of OA self-archiving (4). It is the
smallest of the four factors, but that is unsurprising, as surely
article age and quality are the two biggest determinants of citations,
whether the articles are OA or non-OA. (Perhaps self-citations are the
third biggest contributor). But the OA citing advantage is present for
those self-archived articles, refuting Eysenbach's claim that the
green OA advantage is merely the result of "potential confounds" and
that only the gold OA advantage is real.

I might add that the PLoS Editorial is quite right to say: "Since most
open-access journals are new, comparisons of the effects of open
access with established subscription-based journals are easily
confounded by age and reputation": Comparability and confounding are
indeed major problems for between-journal comparisons, comparing OA
and non-OA journals (gold OA). Until Eysenbach's within-journal PNAS
study, "solid evidence" (for gold OA) was indeed hard to find. But
comparability and confounding are far less of a problem for the
within-journal analyses of self-archiving (green OA), and with them,
solid evidence abounds.

I might further add that the solid pre-existing evidence for the green
OA advantage -- free of the limitations of between-journal comparisons
-- is and always has been, by the same token, evidence for the gold OA
advantage too, for it would be rather foolish and arbitrary to argue
that free accessibility is only advantageous to self-archived
articles, and not to articles published in OA journals!

Yet that is precisely the kind of generalization Eysenbach seems to
want to make (in the opposite direction) in the special case of PNAS
-- a very selective, high-profile, high-impact journal. PNAS articles
that are freely accessible on the PNAS website were found to have a
greater OA advantage than PNAS articles freely accessible only on the
author's website. With just a little reflection, however, it is
obvious that the most likely reason for this effect is the high
profile of PNAS and its website: That effect is hence highly unlikely
to scale to all, most, or even many journals; nor is it likely to
scale in time, for as green OA grows, the green OA harvesters like
OAIster (or even just Google Scholar) will become the natural way and
place to search, not the journal's website.

Having taken up Eysenbach's challenge to test the independence of the
OA self-archiving advantage from "potential confounds," we now
challenge Eysenbach to test the generality of the PNAS gold/green
advantage across the full quality hierarchy of journals, to show it is
not merely a high-end effect.

Let me close by mentioning one variable that Eysenbach did not (and
could not) control for, namely, author self-selection bias (Quality
Bias, QB): His 212 OA authors were asked to rate the relative urgency,
importance, and quality of their articles and there was no difference
between their OA and non-OA articles in these self-ratings. But
(although I myself am quite ready to agree that there was little or no
Quality Bias involved in determining which PNAS authors chose which
PNAS articles to make OA gold), unfortunately these self-ratings are
not likely to be enough to convince the sceptics who interpret the OA
advantage as a Quality Bias (a self-selective tendency to provide OA
to higher quality articles) rather than a Quality Advantage (QA) that
increases the citations of higher quality articles. Not even the prior
evidence of a correlation between earlier downloads and later
citations is enough. The positive result of a more objective test of
Quality Bias (QB) vs. Quality Advantage (QA) (comparing self-selected
vs. mandated self-archiving, and likewise conducted by Chawki Hajjem)
will be reported shortly.


Brody, T., Harnad, S. and Carr, L. (2005) Earlier Web Usage Statistics
as Predictors of Later Citation Impact. Journal of the American
Association for Information Science and Technology (JASIST) 57(8) pp.

Eysenbach G (2006) Citation Advantage of Open Access Articles. PLoS
Biology 4(5) e157 DOI: 10.1371/journal.pbio.0040157

Hajjem, C., Harnad, S. & Gingras, Y. (2005) Ten-Year
Cross-Disciplinary Comparison of the Growth of Open Access and How it
Increases Research Citation Impact. IEEE Data Engineering Bulletin
28(4) pp. 39-47.

Harnad, S. (2006) PLoS, Pipe-Dreams and Peccadillos. PLoS Biology

Harnad, S. (2007) The Open Access Citation Advantage: Quality
Advantage Or Quality Bias? [coming, stay tuned)

MacCallum CJ & Parthasarathy H (2006) Open Access Increases Citation
Rate. PLoS Biol 4(5): e176 DOI: 10.1371/journal.pbio.0040176

Moed, H. F. (2006) The effect of 'Open Access' upon citation impact:
An analysis of ArXiv's Condensed Matter Section

Stevan Harnad
American Scientist Open Access Forum
Received on Wed Jan 17 2007 - 20:14:55 GMT

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