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Copublication improved the dissemination of Cochrane reviews and benefited copublishing journals: a retrospective cohort study

  • Linlin Zhu
    Affiliations
    Department of International Medical Center/Ward of General Practice and National Clinical Research Center for Geriatrics, West China Hospital, Sichuan University, Chengdu, China
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  • Yu Zhang
    Affiliations
    Department of Gastroenterology, The First Affiliated Hospital of Chengdu Medical College, Chengdu, China
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  • Rong Yang
    Affiliations
    Department of International Medical Center/Ward of General Practice and National Clinical Research Center for Geriatrics, West China Hospital, Sichuan University, Chengdu, China
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  • Yang Wu
    Affiliations
    Department of International Medical Center/Ward of General Practice and National Clinical Research Center for Geriatrics, West China Hospital, Sichuan University, Chengdu, China
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  • Yi Lei
    Affiliations
    Department of International Medical Center/Ward of General Practice and National Clinical Research Center for Geriatrics, West China Hospital, Sichuan University, Chengdu, China
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  • Yonggang Zhang
    Correspondence
    Corresponding author: Department of Periodical Press and National Clinical Research Center for Geriatrics, West China Hospital, Sichuan University, Chengdu 610041, China. Tel: +86-199-83137949; fax: +86-28-85582944.
    Affiliations
    Department of Periodical Press and National Clinical Research Center for Geriatrics, West China Hospital, Sichuan University, Chengdu, China
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  • Xiaoyang Liao
    Correspondence
    Corresponding author: International Medical Center / Ward of General Practice and National Clinical Research Center for Geriatrics, West China Hospital, Sichuan University, Chengdu 610041, China. Tel: +86-18-980601359; fax: +86-28-85582944.
    Affiliations
    Department of International Medical Center/Ward of General Practice and National Clinical Research Center for Geriatrics, West China Hospital, Sichuan University, Chengdu, China
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  • Mike Clarke
    Correspondence
    Corresponding author: Northern Ireland Methodology Hub, Centre for Public Health, Queen's University Belfast, Belfast BT12 6BJ, Northern Ireland. Tel: +44-28-90978935; fax: +44-28-90235900.
    Affiliations
    Northern Ireland Methodology Hub, Centre for Public Health, Queen's University Belfast, Belfast, Northern Ireland, UK
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Open AccessPublished:May 29, 2022DOI:https://doi.org/10.1016/j.jclinepi.2022.05.016

      Highlights

      • We conducted the first retrospective cohort study to evaluate the impact of copublication on the citation of Cochrane evidence across a wide range of Cochrane review groups.
      • Copublication of Cochrane reviews in other journals was associated with a higher citation frequency for the evidence and might increase the impact factor of the copublishing journal.
      • Copublication might facilitate broader application of Cochrane evidence and promote its dissemination.
      • Authors may wish to give more consideration to copublishing their Cochrane reviews in another journal.

      Abstract

      Objectives

      To evaluate the impact of copublication on the citation of Cochrane evidence.

      Study Design and Setting

      This was a retrospective cohort study including Cochrane reviews published up to 31 December 2015 and their citing up to 11 July 2021, identified from the Web of Science Core Collection database.

      Results

      A total of 101 copublished and 202 noncopublished Cochrane reviews were included. The median for the total number of citations and the medians for the numbers of citations to the Cochrane review in the first, second, third, and fifth years after publication in the copublished group were higher than those in the noncopublished group [71 (interquartile range {IQR}: 37.5, 118.5) vs. 32.5 (13, 67); 1 (0, 3) vs. 0 (0, 1); 6 (3, 11.5) vs. 2 (1, 5); 8 (4, 15) vs. 3.5 (1, 8); 8 (4, 15) vs. 3 (1, 9), respectively, all P < 0.001]. Copublication of Cochrane reviews meant that 4 of 21 journals and 6 of 22 journals had a higher impact factor in the first and the second year after the copublication than they would have had without the copublication.

      Conclusion

      Copublication is associated with a higher citation frequency of Cochrane reviews and may increase the impact factor of the journal in which it is copublished. This facilitates broader application of Cochrane evidence and promotes its dissemination.

      Keywords

      What is new?

        Key findings

      • This of 101 copublished and 202 matched noncopublished Cochrane reviews found that the median for the total number of citations and the medians for the numbers of citations to the Cochrane review in the first, second, third, and fifth years after publication in the copublished group were higher than those in the noncopublished group.

        What this adds to what was known?

      • Copublication of Cochrane reviews in other journals was associated with a higher citation frequency for the evidence and might increase the impact factor of the copublishing journal.
      • Copublication might facilitate broader application of Cochrane evidence and promote its dissemination.

        What is the implication and what should change now?

      • Authors of Cochrane reviews may wish to give more consideration to copublishing their reviews in another journal.

      1. Introduction

      Since 1994, Cochrane (previously The Cochrane Collaboration) has allowed copublication of Cochrane reviews in specialist journals in certain circumstances [
      • MacLehose H.
      • Mann J.
      Policy and overview of co-publication.
      ]. Copublication of Cochrane reviews is not a duplicate publication or academic misconduct but a way to present the findings and conclusions of the Cochrane review to the readers of a specialist journal to improve access to the Cochrane evidence. The journal version is a secondary publication of the Cochrane review, using the data and interpretation of the version published in the Cochrane Database of Systematic Reviews (CDSR) []. Copublication of Cochrane reviews might be as an abridged version of the full review, in a “Cochrane corner” of a journal (where the summary of the review is accompanied by a commentary) or a short version translated into a language other than English [
      • MacLehose H.
      • Mann J.
      Policy and overview of co-publication.
      ]. Adherence to the formatting requirements of the copublishing journal and the Cochrane copublication policy are required for copublication and the copublished version should be peer-reviewed and edited as per the journal's editorial process. Journals interested in arranging copublication of a Cochrane review need to sign an agreement with Cochrane and, as of July 2021, 82 specialist journals had done so [
      • MacLehose H.
      • Mann J.
      ].
      Copublication of Cochrane reviews in other journals might promote access to the evidence [
      • Greene R.E.
      • Freeman S.R.
      • Lundahl K.
      • Dellavalle R.P.
      Co-publishing Cochrane library systematic reviews: journal editor and Cochrane skin group author experiences.
      ,
      • Wang X.
      • Hawkins B.S.
      • Dickersin K.
      Cochrane systematic reviews and co-publication: dissemination of evidence on interventions for ophthalmic conditions.
      ]. The Cochrane Collaboration intended that the copublished version would meet the same quality standards and would allow the implications of the review to be applied more broadly and would increase dissemination of the information [
      • Pearlman A.S.
      Co-publication-what you always wanted to know but have been afraid to ask!.
      ]. It was also hoped that journals which copublished Cochrane reviews might increase their impact factor (IF) [
      • Greene R.E.
      • Freeman S.R.
      • Lundahl K.
      • Dellavalle R.P.
      Co-publishing Cochrane library systematic reviews: journal editor and Cochrane skin group author experiences.
      ]. Copublication of a shorter version of the Cochrane review might also help those readers who felt challenged by the full-length review. As an extreme example, the Cochrane review with the largest number of included studies (as of May 2022) is a network meta-analysis of drugs to prevent postoperative nausea and vomiting and, if printed, this would fill more than 2,200 pages [
      • Weibel S.
      • Rücker G.
      • Eberhart L.H.
      • Pace N.L.
      • Hartl H.M.
      • Jordan O.L.
      • et al.
      Drugs for preventing postoperative nausea and vomiting in adults after general anaesthesia: a network meta-analysis.
      ], although its copublished versions take up only 11 [
      • Weibel S.
      • Schaefer M.S.
      • Raj D.
      • Rücker G.
      • Pace N.L.
      • Schlesinger T.
      • et al.
      Drugs for preventing postoperative nausea and vomiting in adults after general anaesthesia: an abridged Cochrane network meta-analysis.
      ] and nine pages [
      • Weibel S.
      • Pace N.L.
      • Schaefer M.S.
      • Raj D.
      • Schlesinger T.
      • Meybohm P.
      • et al.
      Drugs for preventing postoperative nausea and vomiting in adults after general anesthesia: an abridged Cochrane network meta-analysis.
      ].
      With so many potential benefits of copublication, it might be expected that copublication would be common. However, for example, only 19.6% of reviews from the Cochrane Eyes and Vision Group were copublished in 2007 and 16.2% in 2014, showing that fewer than one in five of the reviews from this Cochrane review group were copublished and that there was no increase over a decade [
      • Wang X.
      • Hawkins B.S.
      • Dickersin K.
      Cochrane systematic reviews and co-publication: dissemination of evidence on interventions for ophthalmic conditions.
      ,
      • Li T.
      • Scherer R.
      • Twose C.
      • Anton B.
      • Dickersin K.
      Identification and characterization of systematic reviews in eyes and vision. Presented at the XV Cochrane Colloquium, 2007 Oct 23–27, Sao Paulo, Brazil.
      ]. The reasons for this were unclear but might include a lack of awareness of copublication among journal editors, a reluctance by authors to spend time on copublication (especially if the copublished content requires significant changes), a lack of a consensus between Cochrane and other journals on copublication, and the need for different formats [
      • Greene R.E.
      • Freeman S.R.
      • Lundahl K.
      • Dellavalle R.P.
      Co-publishing Cochrane library systematic reviews: journal editor and Cochrane skin group author experiences.
      ].
      Thus, it is important to study the impacts of copublication. The first descriptive study explored the characteristics of copublication in the Cochrane Eyes and Vision Group [
      • Wang X.
      • Hawkins B.S.
      • Dickersin K.
      Cochrane systematic reviews and co-publication: dissemination of evidence on interventions for ophthalmic conditions.
      ]. Wang et al. found that the number of citations for copublished reviews was approximately twice than that of the Cochrane Eyes and Vision Group's other reviews. However, the generalizability of this beyond this one of approximately 50 Cochrane review groups was not known. Therefore, we performed the present study to assess the impact of copublication of Cochrane reviews across all Cochrane review groups. We investigated this in two ways: citations of copublished vs. noncopublished Cochrane reviews and the impact on the IF of the copublishing journal.

      2. Methods

      2.1 Study design and search strategy

      This was a retrospective cohort study of Cochrane copublication based on data collected from the Web of Science Core Collection database. We report it as per the Strengthening the Reporting of Observational Studies in Epidemiology guideline [
      • von Elm E.
      • Altman D.G.
      • Egger M.
      • Pocock S.J.
      • Gøtzsche P.C.
      • Vandenbroucke J.P.
      The strengthening the reporting of observational studies in epidemiology (STROBE) statement: guidelines for reporting observational studies.
      ]. Of the 82 journals which signed a Cochrane copublication agreement [
      • MacLehose H.
      • Mann J.
      Policy and overview of co-publication.
      ], 44 were indexed in this database. We searched the database from inception up to 31 December 2015 using the term “Cochrane” and the names for these 44 journals to identify eligible reviews and up to July 11, 2021 to obtain data on the numbers of citations. The search strategy is shown in Appendix 1. Cochrane reviews which were copublished at the same time or later in journals were the cases (copublication group), although the controls were noncopublished Cochrane reviews from the same Cochrane review group which were published in the same year as the copublished Cochrane review (noncopublication group). We used a case to control ratio of 1:2.

      2.2 Inclusion and exclusion criteria

      Inclusion criteria were (1) the copublished review was published after the publication of the full Cochrane review in the CDSR and (2) the copublished review was based on the Cochrane review published in the CDSR. Exclusion criteria were (1) copublication was a duplicate report; (2) Cochrane reviews that had been updated because the publication that would otherwise have been used; (3) copublication was an editorial, letter, or conference abstract; and (4) a sufficient number of matching Cochrane reviews that had not been copublished was not available.

      2.3 Random sampling

      We used stratified random sampling with proportional allocation within each stratum, where the targeted variables were Cochrane review groups and year of publication of the Cochrane review which had been copublished [
      • Lee J.
      • Lee J.S.
      • Park S.H.
      • Shin S.A.
      • Kim K.
      Cohort profile: the National Health Insurance Service-National Sample cohort (NHIS-NSC), South Korea.
      ,
      • Hwangbo Y.
      • Kang D.
      • Kang M.
      • Kim S.
      • Lee E.K.
      • Kim Y.A.
      • et al.
      Incidence of diabetes after cancer development: a Korean National cohort study.
      ]. First, the sampling strata were the 27 Cochrane review groups with at least one copublished Cochrane review and the 16 publication years (from 2000 to 2015). The Cochrane review groups are listed in Appendix 2. Specifically, all Cochrane reviews published by a Cochrane review group in each year in which they had published a copublished Cochrane review were identified from CDSR. We then excluded any reviews that had been subsequently withdrawn or updated or had been copublished in a journal which was not among the 44 journals with signed agreements with Cochrane which were indexed in the Web of Science Core Collection database. The remaining candidates formed the sampling frame and, within each stratum, we randomly sampled reviews in a 1:2 ratio to match copublished and noncopublished Cochrane reviews. If there were not at least two eligible noncopublished Cochrane reviews for each copublished review, the review was excluded. The sampling procedure was carried out by one author (Y.Z.).

      2.4 Review selection and data extraction

      Decisions about eligibility and data extraction were undertaken independently by two authors (L.L.Z. and Y.W.). Disagreements were resolved by consensus with arbitration by another author (R.Y.) if necessary. We used a standard form to independently extract the following data for eligible reviews: article title, authorship of CDSR and journal version, country of first author, journal of copublication, timing of copublication in relation to publication of the Cochrane review, number of included studies, results and conclusions, and type of copublication. The number of citations to the Cochrane reviews and to the copublished versions and the IF of the copublishing journal were taken from the Web of Science database (Journal Citation Report).

      2.5 Outcomes

      This retrospective cohort study aimed to examine the potential impact of copublication on the promotion and application of evidence-based practice that is based on Cochrane reviews. The primary outcome measures were the mean and median for the total number of citations to the Cochrane reviews and the numbers in the first, second, third year, and fifth year after publication. The secondary outcomes were the ratio of an adjusted IF (excluding the data for the copublished Cochrane review) and the actual IF of the copublishing journal. These two indicators were used to evaluate the impact of copublication on the copublished journal.
      The journal IF is calculated by dividing the number of citations in the current year by the number of articles published in the previous 2 years, which we derived from the Journal Citation Report of Web of Science [
      • Breugelmans J.G.
      • Makanga M.M.
      • Cardoso A.L.
      • Mathewson S.B.
      • Sheridan-Jones B.R.
      • Gurney K.A.
      • et al.
      Bibliometric assessment of European and Sub-Saharan African research output on poverty-related and neglected infectious diseases from 2003 to 2011.
      ]. Thereby, the IF represents the average number of citations for articles published in the past 2 years [
      • Fazel S.
      • Wolf A.
      What is the impact of a research publication?.
      ,
      • Fazel S.
      • Lamsma J.
      Beyond the impact factor?.
      ]. The formula for the IF means that a simultaneous increase/decrease in the number of articles and citations can lead to IF variations in either direction, depending on which variable changes most [
      • Kiesslich T.
      • Weineck S.B.
      • Koelblinger D.
      Reasons for journal impact factor changes: influence of changing source items.
      ,
      • Callaham M.
      • Wears R.L.
      • Weber E.
      Journal prestige, publication bias, and other characteristics associated with citation of published studies in peer-reviewed journals.
      ]. To determine whether the copublications affected the journal's IF, we created an adjusted IF by, first, subtracting the number of citations for the copublished review from the total number of citations (i.e., A) and subtracting the number of copublished reviews from the total number of published articles (i.e., B), A divided by B gives the adjusted IF, which was the IF that the journal would have achieved if it had not copublished the Cochrane review. We then calculated a ratio by dividing this adjusted IF by the journal's actual IF. If this ratio was more than or equal to one, the copublished reviews did not increase the journal IF, although values less than one indicated that the copublished reviews increased the journal IF. If a journal had published more than one copublished review, we calculated a mean ratio of IF and its 95% confidence interval (95% CI) to account for the multiple articles.
      We calculated the number of citations for each copublished review as the sum of the citations to the Cochrane review and to its copublished version, which were derived from the Web of Science database.

      2.6 Statistical analysis

      We used the Pearson’s Chi-squared test to compare categorical variables, the Student’s t-test for continuous and normally distributed variables, and the Mann-Whitney U-test to compare medians if the data were not normally distributed. A P value of <0.05 was considered significant. We used SPSS 25.0 software for the statistical analyses.

      3. Results

      3.1 Baseline characteristics of Cochrane reviews

      Our search for “Cochrane” and the name of one of the 44 eligible journals identified 1,442 publications from the Web of Science Core Collection database up to December 31, 2015, containing 171 possible copublications of Cochrane reviews. After we applied our inclusion and exclusion criteria, 101 of these were eligible for the study. We then randomly sampled 202 of the noncopublished Cochrane reviews in accordance with the methods described above to obtain the 1:2 ratios of cases and controls. The flow diagram is shown in Figure 1. The included reviews were from 27 Cochrane review groups and had been published between 2005 and 2015. The first authors of the copublished reviews and the noncopublished reviews were from 14 countries and 28 countries, respectively. The main characteristics of the two groups of review are shown in Table 1. No significant difference was found in characteristics between the two groups.
      Figure thumbnail gr1
      Fig. 1Flow diagram for identification of the cohort of Cochrane reviews used in this study.
      Table 1Characteristics of the copublished and noncopublished Cochrane reviews
      Study characteristicsCopublished (N = 101)Noncopublished (N = 202)P value
      Year of publication
       2005∼201043861.00
       2011∼201558116
      Authors number, Mean ± SD4.6 ± 1.94.7 ± 2.00.766
      Region of the first author
       European countries751400.669
       Asian countries721
       Oceania countries46
       American countries1535
      In considering any copublication of a Cochrane review (not just those in the 44 journals with signed agreements with Cochrane), we retrieved a total of 8,717 Cochrane reviews from the Web of Science Core Collection database up to December 31, 2015. After removal of duplicates, 6,830 Cochrane reviews remained and we identified copublications for 148 of these between 2005 and 2015. The annual copublication rate in this period fluctuated from 0.96% to 3.94% and there was no clear trend over time (Fig. 2).
      Figure thumbnail gr2
      Fig. 2Annual co-publication rate of Cochrane reviews (2005 to 2015).

      3.2 Characteristics of copublications

      The 101 copublished Cochrane reviews were copublished in 22 of the 44 journals which signed copublication agreements with Cochrane. Twenty eight (27.7%) were published in the journals in the same year as the Cochrane review was published and more than three-quarters of copublications (79.2%) were published within 2 years after the publication of the Cochrane review. The types of copublication were mostly short sections of the Cochrane review (such as the abstract or plain language summary) or an abridged version of the full review (64.4%), with smaller but similar proportions for specially prepared summaries of the review (14.9%) or a “Cochrane corner” (15.8%). The copublication content was usually based on the original Cochrane review (76.2%), with the number of included studies (in 85.1% of the reviews) and the results and conclusion (88.1%) being the same in both versions.
      Only 58 (57.4%) of the copublications clearly contained all the content required in the Cochrane copublication policy [
      • MacLehose H.
      • Mann J.
      Policy and overview of co-publication.
      ], among which six (10.3%) did not place it on the title page. Although a further 24 (23.8%) of the copublications contained relevant information for the corresponding Cochrane review, it was not fully in accordance with the requirements. Characteristics are summarized in Table 2.
      Table 2Characteristics of 101 copublications (journal version)
      Copublications characteristicsN (%)
      Authorship of copublication
       Identical to Cochrane review51 (50.5%)
       Same authors, different order13 (12.9%)
       Authors added or removed37 (36.6%)
      Copublication timing
       Same year as the Cochrane review28 (27.7%)
       Year after publication of the Cochrane review52 (51.5%)
       More than 2 years after the Cochrane review21 (20.8%)
      Copublication content compared to Cochrane review
       Short version
      Short version of a Cochrane review: this includes republishing a part of the review (such as the abstract and plain language summary) or an abridged version [19].
      65 (64.4%)
       Original review summaries
      Review summaries are summaries of a Cochrane review where the authors of the article provide a commentary on the Cochrane review in their own words.
      15 (14.9%)
       Cochrane corner
      Short version alongside an accompanying original commentary/article (referred to as ‘Cochrane corner’) [1].
      16 (15.8%)
       Updated version of the original Cochrane review5 (4.9%)
      Citation of the Cochrane review
       Cited68 (67.3%)
       Not cited33 (32.7%)
      Met the citing requirements for copublication
      This requirement is that a footnote should be placed on the title page of the journal version to inform readers and documenting agencies that the article has been published previously, either in whole or in part, citing the primary publication, for example, “This article is based on a Cochrane Review published in the Cochrane Database of Systematic Reviews (CDSR) YYYY, Issue X, https://doi.org/10.1002/14651858.CD00xxxx (www.cochranelibrary.com provides information). Cochrane reviews are regularly updated as new evidence emerges and in response to feedback and the CDSR should be consulted for the most recent version of the review.” [2].
       Stated the required information58 (57.4%)
      which was a footnote on the title page52 (89.7%)
      which was in another place
      Other places included the background, introduction, conclusion, or acknowledgment section of the article.
      6 (10.3%)
       Did not meet the requirements for copublication content24 (23.8%)
       No detailed description of whether met the requirement19 (18.8%)
      Copublication based on
       Original review77 (76.2%)
       Updated review24 (23.8%)
      Number of included studies
       Same as Cochrane review86 (85.1%)
       More than Cochrane review7 (6.9%)
       Less than Cochrane review8 (7.9%)
      Results and conclusion
       Same as Cochrane review89 (88.1%)
       Different results and conclusion7 (6.9%)
       Different results, same conclusion5 (5.0%)
      a Short version of a Cochrane review: this includes republishing a part of the review (such as the abstract and plain language summary) or an abridged version [].
      b Review summaries are summaries of a Cochrane review where the authors of the article provide a commentary on the Cochrane review in their own words.
      c Short version alongside an accompanying original commentary/article (referred to as ‘Cochrane corner’) [
      • MacLehose H.
      • Mann J.
      Policy and overview of co-publication.
      ].
      d This requirement is that a footnote should be placed on the title page of the journal version to inform readers and documenting agencies that the article has been published previously, either in whole or in part, citing the primary publication, for example, “This article is based on a Cochrane Review published in the Cochrane Database of Systematic Reviews (CDSR) YYYY, Issue X, https://doi.org/10.1002/14651858.CD00xxxx (www.cochranelibrary.com provides information). Cochrane reviews are regularly updated as new evidence emerges and in response to feedback and the CDSR should be consulted for the most recent version of the review.” [].
      e Other places included the background, introduction, conclusion, or acknowledgment section of the article.

      3.3 Number of citations for copublished and noncopublished Cochrane reviews

      The median for the total number citations up to July 2021 was significantly higher in the group of copublished reviews than in the noncopublished group [71 (37.5, 118.5) vs. 32.5 (13, 67), P < 0.001]. Similarly, the median number of citations in the first year, second year, third year, and fifth year after publication in the copublished group was higher than in the noncopublished group [1 (0, 3) vs. 0 (0, 1), 6 (3, 11.5) vs. 2 (1, 5), 8 (4, 15) vs. 3.5 (1, 8), 8 (4, 15) vs. 3 (1, 9), respectively, all P < 0.001] (Table 3).
      Table 3Total number of citations for the copublished and noncopublished reviews
      Average number of citationsCopublished (N = 101)Noncopublished (N = 202)P value
      Current total number of citations
       Mean ± SD93.5 ± 82.858.9 ± 78.3<0.001
       Median (IQR)71 (37.5, 118.5)32.5 (13, 67)<0.001
      Year of publication
       Mean ± SD2.2 ± 3.10.8 ± 1.8<0.001
       Median (IQR)1 (0, 3)0 (0, 1)<0.001
      Second year after publication
       Mean ± SD8.3 ± 8.43.6 ± 5.3<0.001
       Median (IQR)6 (3, 11.5)2 (1, 5)<0.001
      Third year after publication
       Mean ± SD11.2 ± 10.26.0 ± 7.6<0.001
       Median (IQR)8 (4, 15)3.5 (1, 8)<0.001
      Fifth year after publication
       Mean ± SD11.9 ± 12.16.3 ± 7.5<0.001
       Median (IQR)8 (4, 15)3 (1, 9)<0.001
      Abbreviation: IQR, interquartile range.

      3.4 Effect on journal impact factor

      The 101 included Cochrane reviews were copublished in 22 journals, one of which did not have an IF until the second year after its copublication of a Cochrane review. Among these journals, the ratio of the adjusted IF to the actual IF was less than one in four (19.0%) of 21 journals for the first year after publication. In the second year after publication of the copublished review, the ratio was less than one in six (27.3%) of 22 journals. This means that the copublished review increased the IF for these four and six journals in the first and second year after publication of their version of the Cochrane review. The results for all journals are shown in Table 4.
      Table 4Ratio of the adjusted IF
      actual IF: Total number of citations/total number of published articles.
      to the actual IF
      adjusted IF: Total number of citations—number of citations for the copublished review/Total number of published articles—number of copublished reviews.
      Journal nameRatio—first year after copublishedRatio—second year after copublished
      Mean ± SD (95% CI)Mean ± SD (95% CI)
      American Journal of Hypertension0.978 ± 0.003 (0.946-1.009)0.973 ± 0.001 (0.960-0.986)
      American Journal of Kidney Diseases0.999 ± 0.001 (0.997-1.002)0.999 ± 0.001 (0.996-1.003)
      Anesthesia and Analgesia0.998
      If there was only one result (IF ratio) for a journal, we could not calculate an SD and 95% CI.
      0.997
      Australian Occupational Therapy Journal1.0101.009
      BJU International1.000 ± 0.000 (0.999-1.001)1.000 ± 0.000 (0.999-1.001)
      British Journal of Dermatology1.001 ± 0.000 (1.000-1.002)1.001 ± 0.000 (1.000-1.002)
      Clinical Orthopedics and Related Research1.006 ± 0.000 (1.006-1.007)1.007 ± 0.000 (1.007-1.007)
      Clinical Otolaryngology1.012 ± 0.003 (0.980-1.043)1.009 ± 0.007 (0.926-1.091)
      Colorectal DiseaseNone
      Colorectal disease did not have an IF until 2007 and the copublished review was published in 2005.
      0.980
      Developmental Medicine & Child Neurology0.9970.992
      Diabetic Medicine1.001 ± 0.002 (0.994-1.008)0.998 ± 0.001 (0.996-1.001)
      European Journal of Physical and Rehabilitation Medicine1.015 ± 0.003 (1.009-1.021)1.009 ± 0.004 (1.001-1.016)
      International Journal of Nursing Studies1.001 ± 0.002 (0.996-1.006)1.002 ± 0.002 (0.997-1.006)
      Journal of Pediatrics and Child Health1.0031.003
      Journal of Rehabilitation Medicine0.9950.998
      Journal of Rheumatology1.005 ± 0.001 (1.002-1.008)1.005 ± 0.001 (1.002-1.008)
      Journal of the American Academy of Dermatology1.002 ± 0.001 (0.995-1.008)1.001 ± 0.001 (0.994-1.007)
      Revista Portuguesa de Cardiologia1.006 ± 0.001 (1.002-1.011)1.008 ± 0.002 (1.002-1.013)
      Spinal Cord0.9990.993
      Spine0.999 ± 0.001 (0.998-1.001)0.998 ± 0.001 (0.996-1.000)
      Stroke1.0011.001
      Techniques in Coloproctology0.993 ± 0.011 (0.853-1.132)0.995 ± 0.009 (0.880-1.109)
      a Colorectal disease did not have an IF until 2007 and the copublished review was published in 2005.
      b If there was only one result (IF ratio) for a journal, we could not calculate an SD and 95% CI.
      c actual IF: Total number of citations/total number of published articles.
      d adjusted IF: Total number of citations—number of citations for the copublished review/Total number of published articles—number of copublished reviews.
      If the ratio of the two IF is less than 1.00, this indicates that the inclusion of the copublished Cochrane review led to a higher IF for the journal than would otherwise be the case.

      4. Discussion

      The present study was designed with the aim of exploring the potential impact of the copublication of Cochrane reviews. To our knowledge, this is the first retrospective cohort study investigating copublication that covers multiple disciplines and multiple Cochrane review groups. Our results show that the number of citations for copublished Cochrane reviews was significantly higher than that for noncopublished reviews both in total citations and in citations in the first 5 years after publication. In addition, the IF for four of 21 journals was raised by the publication and citation of a version of a Cochrane review in the first year after its publication and in six of 22 journals in the second year. Together, these findings suggest that copublication of Cochrane reviews not only improved the dissemination and accessibility of Cochrane evidence but also facilitated its uptake.
      For the period of our study, the proportion of Cochrane reviews that were copublished varied year on year (0.96% to 3.94%), with no clear trend over time. These proportions were well below that found for one individual Cochrane review group [
      • Wang X.
      • Hawkins B.S.
      • Dickersin K.
      Cochrane systematic reviews and co-publication: dissemination of evidence on interventions for ophthalmic conditions.
      ,
      • Li T.
      • Scherer R.
      • Twose C.
      • Anton B.
      • Dickersin K.
      Identification and characterization of systematic reviews in eyes and vision. Presented at the XV Cochrane Colloquium, 2007 Oct 23–27, Sao Paulo, Brazil.
      ] and the reasons for this are unclear [
      • Greene R.E.
      • Freeman S.R.
      • Lundahl K.
      • Dellavalle R.P.
      Co-publishing Cochrane library systematic reviews: journal editor and Cochrane skin group author experiences.
      ]. Although some authors and editors might be concerned that copublication was an academic misconduct or duplicated publication, this was not the case. As per the International Committee of Medical Journal Editors (ICMJE) recommendations, duplicate publication is publication of an article that overlaps substantially with one already published, without clear, visible reference to the previous publication [
      Recommendations for the Conduct, reporting, editing, and publication of scholarly work in medical journals.
      ]. Moreover, ICMJE has proposed a conceptual framework of ‘acceptable secondary publication’. Specifically, secondary publication or copublication is permitted as a means to disseminate important information but several conditions need to be met [
      Recommendations for the Conduct, reporting, editing, and publication of scholarly work in medical journals.
      ], such as approval from the editors of both journals, an intention that the secondary version is for a different group of readers, and a clear indication that the secondary version is based on an article that has been published elsewhere. On this basis, Cochrane developed a series of policies to promote copublication [
      • MacLehose H.
      • Mann J.
      Policy and overview of co-publication.
      ], which are consistent with the guidance of ICMJE, and therefore, in keeping with good practice.
      We have shown that the number of citations of copublished Cochrane reviews was significantly higher than those of noncopublished Cochrane reviews, which is consistent with a previous study [
      • Wang X.
      • Hawkins B.S.
      • Dickersin K.
      Cochrane systematic reviews and co-publication: dissemination of evidence on interventions for ophthalmic conditions.
      ]. This increased number of citations suggests benefits for the Cochrane evidence. First, each copublishing journal had a specialist audience, thereby bringing the findings of the Cochrane review to that audience [
      • Campbell P.
      Escape from the impact factor.
      ,
      • Neylon C.
      • Wu S.
      Article-level metrics and the evolution of scientific impact.
      ,
      • Haven T.L.
      • Bouter L.M.
      • Smulders Y.M.
      • Tijdink J.K.
      Perceived publication pressure in Amsterdam: survey of all disciplinary fields and academic ranks.
      ] and some previous studies have shown that meta-analyses had higher rates of citation in specialty journals [
      • Bhandari M.
      • Busse J.
      • Devereaux P.J.
      • Montori V.M.
      • Swiontkowski M.
      • Tornetta III, P.
      • et al.
      Factors associated with citation rates in the orthopedic literature.
      ,
      • Willis D.L.
      • Bahler C.D.
      • Neuberger M.M.
      • Dahm P.
      Predictors of citations in the urological literature.
      ,
      • Winnik S.
      • Raptis D.A.
      • Walker J.H.
      • Hasun M.
      • Speer T.
      • Clavien P.A.
      • et al.
      From abstract to impact in cardiovascular research: factors predicting publication and citation.
      ,
      • Lee K.E.
      • McMullen N.
      • Kota H.
      • Peterson K.
      • Oravec C.
      • Frey C.
      • et al.
      Predictors of citations in neurosurgical research: a 5-year follow-up.
      ]. Second, the higher number of citations suggests that the corresponding Cochrane review would have received more attention because of the copublication. Third, the copublished version might make the Cochrane evidence more accessible to potential users than the full Cochrane review and might be more useful to them as a reference source [
      • Pearlman A.S.
      Co-publication-what you always wanted to know but have been afraid to ask!.
      ].
      This was the first study which examined the impact of copublication on the copublishing journals. It investigated the effect of the copublication on the journal's IF and found that four of 21 journals had a higher IF in the first year after publication and six of 22 journals had a higher IF in the second year because of the copublished Cochrane review. The longer-term effects were less clear because of the way in which the IF was calculated but citation rate peaks vary across different journals, ranging from 2.5 to 7.2 years after publication [
      • Walters G.D.
      The citation life cycle of articles published in 13 American Psychological Association journals: a 25-year longitudinal analysis.
      ,
      • Madhugiri V.S.
      • Sasidharan G.M.
      • Subeikshanan V.
      • Dutt A.
      • Ambekar S.
      • Strom S.F.
      An analysis of the citation climate in neurosurgical literature and description of an interfield citation metric.
      ], which suggest that journals might benefit from copublication beyond the 2 years used to calculate their IF [
      • Roldan-Valadez E.
      • Salazar-Ruiz S.Y.
      • Ibarra-Contreras R.
      • Rios C.
      Current concepts on bibliometrics: a brief review about impact factor, Eigenfactor score, CiteScore, SCImago Journal Rank, Source-Normalised Impact per Paper, H-index, and alternative metrics.
      ]. This is an area for further research. Furthermore, it has been acknowledged that comparing IF across different areas of research might not be meaningful [
      • Kianifar H.
      • Sadeghi R.
      • Zarifmahmoudi L.
      Comparison between impact factor, eigenfactor metrics, and SCimago journal rank indicator of pediatric neurology journals.
      ,
      • Ascaso F.J.
      [Impact factor, eigenfactor and article influence].
      ] and, because the 27 Cochrane review groups with data in this study represented a wide range of areas, the size of any impact on a journal's IF might vary for different disciplines.
      This study showed that Cochrane's copublication policies [
      • MacLehose H.
      • Mann J.
      Policy and overview of co-publication.
      ], including description of copublication type and placing of a footnote on the title page of the journal version were not fulfilled by all authors. Among the group of 101 copublished reviews, only two-thirds (68, 67.3%) of the copublications explicitly cited the original Cochrane review and only 58 (57.4%) of the copublications provided the information in a footnote on the title page of the journal version. Citation of the corresponding Cochrane review and a prominent position of this information are important for showing readers of the copublished articles to find the original Cochrane review.
      Our study has some limitations. First, we only incorporated data on citations and copublications from the Web of Science Core Collection database. Second, some Cochrane reviews published an updated version in a later year, which we excluded from this study, which might lead to selection bias. Third, there might be selection bias between specific copublications and noncopublications but this should not affect our results or conclusions because journals and editors have a variety of priorities for publishing articles. Fourth, although some studies have found that meta-analyses have higher rates of citation than other articles, which might make it more likely that copublishing a version of Cochrane containing meta-analyses would have more impact on a journal's IF than copublishing any other type of article; other studies have argued that study design does not significantly influence citation rate [
      • Callaham M.
      • Wears R.L.
      • Weber E.
      Journal prestige, publication bias, and other characteristics associated with citation of published studies in peer-reviewed journals.
      ,
      • Sochacki K.R.
      • Jack 2nd, R.A.
      • Nauert R.
      • Harris J.D.
      Correlation between quality of evidence and number of citations in top 50 cited articles in Rotator Cuff Repair Surgery.
      ]. For example, there may also be important associations with the number of authors [
      • Oravec C.S.
      • Frey C.D.
      • Berwick B.W.
      • Vilella L.
      • Aschenbrenner C.A.
      • Wolfe S.Q.
      • et al.
      Predictors of citations in neurosurgical research.
      ,
      • Harsh D.
      • Adnan H.S.
      • Raees A.P.
      • Manjul T.
      • Anil N.
      How many neurosurgeons does it take to author an article and what are the other factors that impact citations?.
      ,
      • Ponce F.A.
      • Lozano A.M.
      Highly cited works in neurosurgery. Part I: the 100 top-cited papers in neurosurgical journals.
      ] or their geographic location [
      • Yom K.H.
      • Jenkins N.W.
      • Parrish J.M.
      • Brundage T.S.
      • Hrynewycz N.M.
      • Narain A.S.
      • et al.
      Predictors of citation rate in the spine literature.
      ]. These issues should be investigated in further research.
      In conclusion, we have shown that copublication increases the number of citations for Cochrane reviews and may increase the IF of the copublishing journal. This facilitates a broader application of Cochrane evidence and promotes its dissemination. On this basis, we encourage Cochrane authors to copublish their reviews and we believe that copublication may be a “win-win-win” for practitioners, journals, and patients.

      CRediT authorship contribution statement

      Linlin Zhu: Formal analysis, Investigation, Visualization, Writing – original draft. Yu Zhang: Methodology, Visualization, Data curation. Rong Yang: Methodology, Software. Yang Wu: Validation, Investigation. Yi Lei: Resources, Data curation. Yonggang Zhang: Conceptualization, Project administration, Supervision, Writing – review & editing. Xiaoyang Liao: Project administration, Supervision. Mike Clarke: Project administration, Supervision, Writing – review & editing.

      Appendix A. Supplementary Data

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