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

Stevan Harnad harnad at ECS.SOTON.AC.UK
Wed Jan 17 14:24:19 EST 2007

    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

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