The Open Access Citation Advantage: Quality Advantage Or Quality Bias?

Stevan Harnad harnad at ECS.SOTON.AC.UK
Sun Jan 21 05:53:54 EST 2007

    Full version with figures and hyperlinks:

The Open Access Citation Advantage: Quality Advantage Or Quality Bias?

    SUMMARY: Many studies have now reported the positive correlation
    between Open Access (OA) self-archiving and citation counts ("OA
    Advantage," OAA). But does this OAA occur because (QB) authors are
    more likely to self-selectively self-archive articles that are
    more likely to be cited (self-selection "Quality Bias": QB)? or
    because (QA) articles that are self-archived are more likely
    to be cited ("Quality Advantage": QA)? The probable answer is
    both. Three studies [by (i) Kurtz and co-workers in astrophysics,
    (ii) Moed in condensed matter physics, and (iii) Davis & Fromerth
    in mathematics] had reported the OAA to be due to QB [plus Early
    Advantage, EA, from self-archiving the preprint before publication,
    in (i) and (ii)] rather than QA. These three fields, however, (1)
    have less of a postprint access problem than most other fields and
    (i) and (ii) also happen to be among the minority of fields that (2)
    make heavy use of prepublication preprints. Chawki Hajjem has now
    analyzed preliminary evidence based on over 100,000 articles from
    multiple fields, comparing self-selected self-archiving with mandated
    self-archiving to estimate the contributions of QB and QA to the OAA.
    Both factors contribute, and the contribution of QA is greater.

This is a preview of some preliminary data (not yet refereed), collected
by my doctoral student at UQaM, Chawki Hajjem. 

This study was done in part by way of response to Henk Moed's replies
to my comments on Moed's (self-archived) preprint:

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

Moed's study is about the "Open Access Advantage" (OAA) -- the higher
citation counts of self-archived articles -- observable across
disciplines as well as across years (red bars are the OAA):

    FIGURE 1. Open Access Citation Advantage By Discipline and By Year.
    Green bars are percentage of articles self-archived (%OA); red bars,
    percentage citation advantage (%OAA) for self-archived articles
    for 10 disciplines (upper chart) across 12 years (lower chart,
    1992-2003). Gray curve indicates total articles by discipline
    and year.  Source: Hajjem, C., Harnad, S. and 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.

The focus of the present discussion is the factors underlying the OAA.
There are at least five potential contributing factors, but only
three of them are under consideration here: (1) Early Advantage (EA),
(2) Quality Advantage (QA) and (3) Quality Bias (QB -- also called
"Self-Selection Bias").

Preprints that are self-archived before publication have an Early
Advantage (EA): they get read, used and cited earlier. This is

    Kurtz, Michael and Brody, Tim (2006) The impact loss to authors
    and research. In, Jacobs, Neil (ed.) Open Access: Key strategic,
    technical and economic aspects. Oxford, UK, Chandos Publishing.

In addition, the proportion of articles self-archived at or after
publication is higher in the higher "citation brackets": the more highly
cited articles are also more likely to be the self-archived articles.

    FIGURE 2. Correlation between Citedness and Ratio of Open Access (OA)
    to Non-Open Access (NOA) Ratios.  The (OAc/TotalOAc)/(NOAc/TotalNOAc)
    ratio (across all disciplines and years) increases as citation count
    (c) increases (r = .98, N=6, p<.005).  The more cited an article,
    the more likely that it is OA. (Hajjem et al.  2005)

The question, then, is about causality: Are self-archived articles more
likely to be cited because they are self-archived (QA)? Or are articles
more likely to be self-archived because they are more likely to be cited

The most likely answer is that both factors, QA and QB, contribute to
the OAA: the higher quality papers gain more from being made more
accessible (QA: indeed the top 10% of articles tend to get 90% of the
citations). But the higher quality papers are also more likely to be
self-archived (QB).

As we will see, however, the evidence to date, because it has been based
exclusively on self-selected (voluntary) self-archiving, is equally
compatible with (i) an exclusive QA interpretation, (ii) an exclusive QB
interpretation or (iii) the joint explanation that is probably the
correct one.

The only way to estimate the independent contributions of QA and QB is
to compare the OAA for self-selected (voluntary) self-archiving with the
OAA for imposed (obligatory) self-archiving. We report some preliminary
results for this comparison here, based on the (still small sample of)
Institutional Repositories that already have self-archiving mandates
(chiefly CERN, U. Southampton, QUT, U. Minho, and U. Tasmania).

    FIGURE 3. Self-Selected Self-Archiving vs. Mandated Self-Archiving:
    Within-Journal Citation Ratios (for 2004, all fields).  S = citation
    counts for articles self-archived at institutions with (Sm) and
    without (Sn) a self-archiving mandate. N = citation counts for
    non-archived articles at institutions with (Nm) and without (Nn)
    mandate (i.e., Nm = articles not yet compliant with mandate). Grand
    average of (log) S/N ratios (106,203 articles; 279 journals) is the OA
    advantage (18%); this is about the same as for Sn/Nn (27972 articles,
    48 journals, 18%) and Sn/N (17%); ratio is higher for Sm/N (34%),
    higher still for Sm/Nm (57%, 541 articles, 20 journals); and Sm/Sn =
    27%, so self-selected self-archiving does not yield more citations
    than mandated; rather the reverse. (All six within-pair differences
    are significant: correlated sample t-tests.) (NB: preliminary,
    unrefereed results.)

Summary: These preliminary results suggest that both QA and QB
contribute to OAA, and that the contribution of QA is greater than that
of QB.

Discussion: On Fri, 8 Dec 2006, Henk Moed [HM] wrote: 

>>> HM: "Below follow some replies to your comments on my preprint 'The
>>> effect of 'Open Access' upon citation impact: An analysis of ArXiv's
>>> Condensed Matter Section'...
>>> "1. Early view effect. [EA] In my case study on 6 journals in the field
>>> of condensed matter physics, I concluded that the observed differences
>>> between the citation age distributions of deposited and non-deposited
>>> ArXiv papers can to a large extent - though not fully - be explained by
>>> the publication delay of about six months of non-deposited articles
>>> compared to papers deposited in ArXiv. This outcome provides evidence
>>> for an early view [EA] effect upon citation impact rates, and
>>> consequently upon ArXiv citation impact differentials (CID, my term) or
>>> Arxiv Advantage (AA, your term)."
>> SH: "The basic question is this: Once the AA (Arxiv Advantage) has been
>> adjusted for the "head-start" component of the EA (by comparing articles
>> of equal age -- the age of Arxived articles being based on the date of
>> deposit of the preprint rather than the date of publication of the
>> postprint), how big is that adjusted AA, at each article age? For that
>> is the AA without any head-start. Kurtz never thought the EA component
>> was merely a head start, however, for the AA persists and keeps growing,
>> and is present in cumulative citation counts for articles at every age
>> since Arxiving began".
> HM: "Figure 2 in the interesting paper by Kurtz et al. (IPM, v. 41, p.
> 1395-1402, 2005) does indeed show an increase in the very short term
> average citation impact (my terminology; citations were counted during
> the first 5 months after publication date) of papers as a function of
> their publication date as from 1996. My interpretation of this figure is
> that it clearly shows that the principal component of the early view
> effect is the head-start: it reveals that the share of astronomy papers
> deposited in ArXiv (and other preprint servers) increased over time.
> More and more papers became available at the date of their submission to
> a journal, rather than on their formal publication date. I therefore
> conclude that their findings for astronomy are fully consistent with my
> outcomes for journals in the field of condensed matter physics."

The findings are definitely consistent for Astronomy and for Condensed
Matter Physics. In both cases, most of the observed OAA came from the
self-archiving of preprints before publication (EA).

Moreover, in Astronomy there is already 100% "OA" to all articles after
publication, and this has been the case for years now (for the reasons
Michael Kurtz and Peter Boyce have pointed out: all research-active
astronomers have licensed access as well as free ADS access to all of
the closed circle of core Astronomy journals: otherwise they simply
cannot be research-active). This means that there is only room for EA in
Astronomy's OAA. And that means that in Astronomy all the questions
about QA vs QB (self-selection bias) apply only to the self-archiving of
prepublication preprints, not to postpublication postprints, which are
all effectively "OA."

To a lesser extent, something similar is true in Condensed-Matter
Physics (CondMP): In general, research-active physicists have better
access to their required journals via online licensing than other fields
do (though one does wonder about the "non-research-active" physicists,
and what they could/would do if they too had OA!). And CondMP too is a
preprint self-archiving field, with most of the OAA differential again
concentrated on the prepublication preprints (EA). Moreover, Moed's test
for whether or not a paper was self-archived was based entirely on its
presence/absence in ArXiv (as opposed to elsewhere on the Web, e.g., on
the author's website or in the author's Institutional Repository).

Hence Astronomy and CondMP are fields that are "biassed" toward EA
effects. It is not surprising, therefore, that the lion's share of the
OAA turns out to be EA in these fields. It also means that the remaining
variance available for testing QA vs. QB in these fields is much
narrower than in fields that do not self-archive preprints only, or

Hence there is no disagreement (or surprise) about the fact that most of
the OAA in Astronomy and CondMP is due to EA. (Less so in the
slower-moving field of maths; see: "Early Citation Advantage?.")

>> SH: "The fact that highly-cited articles (Kurtz) and articles by
>> highly-cited authors (Moed) are more likely to be Arxived certainly does
>> not settle the question of cause and effect: It is just as likely that
>> better articles benefit more from Arxiving (QA) as that better
>> authors/articles tend to Arxive/be-Arxived more (QB)."
> HM: "2. Quality bias. I am fully aware that in this research context one
> cannot assess whether authors publish [sic] their better papers in the
> ArXiv merely on the basis of comparing citation rates of archived and
> non-archived papers, and I mention this in my paper. Citation rates may
> be influenced both by the 'quality' of the papers and by the access
> modality (deposited versus non-deposited). This is why I estimated
> author prominence on the basis of the citation impact of their
> non-archived articles only. But even then I found evidence that
> prominent, influential authors (in the above sense) are overrepresented
> in papers deposited in ArXiv."

I agree with all this: The probable quality of the article was estimated
from the probable quality of the author, based on citations for non-OA
articles. Now, although this correlation, too, goes both ways (are
authors' non-OA articles more cited because their authors self-archive
more or do they self-archive more because they are more cited?), I do
agree that the correlation between self-archiving-counts and
citation-counts for non-self-archived articles by the same author is
more likely to be a QB effect. The question then, of course, is: What
proportion of the OAA does this component account for?

> HM: "But I did more that that. I calculated Arxiv Citation Impact
> Differentials (CID, my term, or ArXiv Advantage, AA, your term) at the
> level of individual authors. Next, I calculated the median CID over
> authors publishing in a journal. How then do you explain my empirical
> finding that for some authors the citation impact differential (CID) or
> ArXiv Advantage is positive, for others it is negative, while the median
> CID over authors does not significantly differ from zero (according to a
> Sign test) for all journals studied in detail except Physical Review B,
> for which it is only 5 per cent? If there is a genuine 'OA advantage' at
> stake, why then does it for instance not lead to a significantly
> positive median CID over authors? Therefore, my conclusion is that,
> controlling for quality bias and early view effect, in the sample of 6
> journals analysed in detail in my study, there is no sign of a general
> 'open access advantage' of papers deposited in ArXiv's Condensed Matter
> Section."

My interpretation is that EA is the largest contributor to the OAA in
this preprint-intensive field (i.e., most of the OAA comes from the
prepublication component) and that there is considerable variability in
the size of the (small) residual (non-EA) OAA. For a small sample, at
the individual journal level, there is not enough variance left for a
significant OAA, once one removes the QB component too. Perhaps this is
all that Henk Moed wished to imply. But the bigger question for OA
concerns all fields, not just those few that are preprint-intensive and
that are relatively well-heeled for access to the published version.
Indeed, the fundamental OA and OAA questions concern the postprint (not
the preprint) and the many disciplines that do have access problems, not
the happy few that do not!

The way to test the presence and size of both QB and QA in these non-EA
fields is to impose the OA, preferably randomly, on half the sample, and
then compare the size of the OAA for imposed ("mandated") self-archiving
(Sm) with the size of the OAA for self-selected ("nonmandated")
self-archiving (Sn), in particular by comparing their respective ratios
to non-self-archived articles in the same journal and year: Sm/N vs.

If Sn/N > Sm/N then QB > QA, and vice versa. If Sn/N = 1, then QB is 0.
And if Sm/N = 1 then QA is 0.

It is a first approximation to this comparison that has just been done
(FIGURE 3) by my doctoral student, Chawki Hajjem, across fields, for
self-archived articles in five Institutional Repositories (IRs) that
have OA self-archiving mandates, for 106,203 articles published in 276
biomedical journal 2004, above.

The mandates are still very young and few, hence the sample is still
small; and there are many potential artifacts, including selective
noncompliance with the mandate as well as disciplinary bias. But the
preliminary results so far suggest that (1) QA is indeed > 0, and (2) QA
> QB. 

[I am sure that we will now have a second round from die-hards who will
want to argue for a selective-compliance effect, as a 2nd-order last
gasp for the QB-only hypothesis, but of course that loses all
credibility as IRs approach 100% compliance: We are analyzing our
mandated IRs separately now, to see whether we can detect any trends
correlated with an IR's %OA. But (except for the die-hards, who will
never die), I think even this early sample already shows that the OA
advantage is unlikely to be only or mostly a QB effect.]

> HM: "3. Productive versus less productive authors. My analysis of
> differences in Citation Impact differentials between productive and less
> productive authors may seem "a little complicated". My point is that if
> one selects from a set of papers deposited in ArXiv a paper authored by
> a junior (or less productive) scientist, the probability that this paper
> is co-authored by a senior (or more productive) author is higher than it
> is for a paper authored by a junior scientist but not deposited in
> ArXiv. Next, I found that papers co-authored by both productive and less
> productive authors tend to have a higher citation impact than articles
> authored solely by less productive authors, regardless of whether these
> papers were deposited in ArXiv or not. These outcomes lead me to the
> conclusion that the observed higher CID for less productive authors
> compared to that of productive authors can be interpreted as a quality
> bias."

It still sounds a bit complicated, but I think what you mean is that (1)
mixed multi-author papers (ML, with M = More productive authors, L =
less productive authors) are more likely to be cited than unmixed
multi-author (LL) papers with the same number of authors, and that (2)
such ML papers are also more likely to be self-archived. (Presumably MM
papers are the most cited and most self-archived of multi-author

That still sounds to me like a variant on the citation/self-archiving
correlation, and hence intepretable as either QA or QB or both. (Chawki
Hajjem has also found that citation counts are positively correlated
with the number of authors an article has: this could either be a
self-citation bias or evidence that multi-authored paper tend to be
better ones.)

> HM: "4. General comments. In the citation analysis by Kurtz et al.
> (2005), both the citation and target universe contain a set of 7 core
> journals in astronomy. They explain their finding of no apparent OA
> effect in his study of these journals by postulating that "essentially
> all astronomers have access to the core journals through existing
> channels". In my study the target set consists of a limited number of
> core journals in condensed matter physics, but the citation universe is
> as large as the total Web of Science database, including also a number
> of more peripherical journals in the field. Therefore, my result is
> stronger than that obtained by Kurtz at al.: even in this much wider
> citation universe, I do not find evidence for an OA advantage effect."

I agree that CondMP is less preprint-intensive, less accessible and less
endogamous than Astrophysics, but it is still a good deal more
preprint-intensive and accessible than most fields (and I don't yet know
what role the exogamy/enodgamy factor plays in either citations or the
OAA: it will be interesting to study, among many other candidate
metrics, once the entire literature is OA).

> HM: "I realize that my study is a case study, examining in detail 6
> journals in one subfield. I fully agree with your warning that one
> should be cautious in generalizing conclusions from case studies, and
> that results for other fields may be different. But it is certainly not
> an unimportant case. It relates to a subfield in physics, a discipline
> that your pioneering and stimulating work (Harnad and Brody, D-Lib Mag.,
> June 2004) has analysed as well at a more aggregate level. I hope that
> more case studies will be carried out in the near future, applying the
> methodologies I proposed in my paper."

Your case study is very timely and useful. However, robot-based studies
based on much larger samples of journals and articles have now confirmed
the OAA in many more fields, most of them not preprint-based at all, and
with access problems more severe than those of physics. 


I would like to conclude with a summary of the "QB vs. QA" evidence to
date, as I understand it:

(1) Many studies have reported the OA Advantage, across many fields.

(2) Three studies have reported QB in preprint-intensive fields that
have either no postprint access problem or markedly less than other
fields (astrophysics, condensed matter, mathematics).

(3) The author of one of these three studies is pro-OA (Kurtz, who is
also the one who drew my attention to the QA counterevidence); the
author of the second is neutral (Moed); and the author of the third
might (I think -- I'm not sure) be mildly anti-OA (Davis -- now
collaborating with a publisher to do a 4-year [sic!] long-term study on
QA vs QB).

    Henneken, E. A., Kurtz, M. J., Eichhorn, G., Accomazzi, A., Grant,
    C., Thompson, D., and Murray, S. S. (2006) Effect of E-printing
    on Citation Rates in Astronomy and Physics. Journal of Electronic
    Publishing, Vol.  9, No. 2, Summer 2006

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

    Davis, P. M. and Fromerth, M. J. (2007) Does the arXiv lead to higher
    citations and reduced publisher downloads for mathematics articles?
    Scientometics, accepted for publication. See critiques: 1, 2

(4) So the overall research motivation for testing QB is not an anti-OA

(5) On the other hand, the motivation on the part of some publishers to
put a strong self-serving spin on these three QB findings is of course
very anti-OA and especially, now, anti-OA-self-archiving-mandate.
(That's quite understandable, and no problem at all.)

(6) In contrast to the three studies that have reported what they
interpret as evidence of QB (Kurtz in astro, Moed in cond-mat and Davis
in maths), there are the many other studies that report large OA
citation (and download) advantages, across a large number of fields.
Those who have interests that conflict with OA and OA self-archiving
mandates are ignoring or discounting this large body of studies, and
instead just spinning the three QB reports as their justification for
ignoring the larger body of findings.

This will all be resolved soon, and the outcome of our QA vs. QB
comparison for mandated vs. self-selected self-archiving already heralds
this resolution. I am pretty confident that the empirical facts will
turn out to have been the following: Yes, there is a QB component in the
OA advantage (especially in the preprinting fields, such as astro,
cond-mat and maths). But that QB component is neither the sole factor
nor the largest factor in the OA advantage, particularly in the
non-preprint fields with access problems -- and those fields constitute
the vast majority. That will be the outcome that is demonstrated, and
eventually not only the friends of OA but the foes of OA will have no
choice but to acknowledge the new reality of OA, its benefits to
research and researchers, and its immediate reachability through the
prompt universal adoption of OA self-archiving mandates.

Stevan Harnad

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