Publication bias in gastroenterological research – a retrospective cohort study based on abstracts submitted to a scientific meeting
© Timmer et al; licensee BioMed Central Ltd. 2002
Received: 27 October 2001
Accepted: 26 April 2002
Published: 26 April 2002
The aim of this study was to examine the determinants of publication and whether publication bias occurred in gastroenterological research.
A random sample of abstracts submitted to DDW, the major GI meeting (1992–1995) was evaluated. The publication status was determined by database searches, complemented by a mailed survey to abstract authors. Determinants of publication were examined by Cox proportional hazards model and multiple logistic regression.
The sample included abstracts on 326 controlled clinical trials (CCT), 336 other clinical research reports (OCR), and 174 basic science studies (BSS). 392 abstracts (47%) were published as full papers. Acceptance for presentation at the meeting was a strong predictor of subsequent publication for all research types (overall, 54% vs. 34%, OR 2.3, 95% CI 1.7 to 3.1). In the multivariate analysis, multi-center status was found to predict publication (OR 2.8, 95% CI 1.6–4.9). There was no significant association between direction of study results and subsequent publication. Studies were less likely to be published in high impact journals if the results were not statistically significant (OR 0.5, 95 CI 95% 0.3–0.6). The author survey identified lack of time or interest as the main reason for failure to publish.
Abstracts which were selected for presentation at the DDW are more likely to be followed by full publications. The statistical significance of the study results was not found to be a predictor of publication but influences the chances for high impact publication.
Publication bias is the tendency to submit or accept manuscripts for publication based on the direction or strength of the study findings . There are concerns that studies with statistically significant results are more likely to be published compared to studies with non significant (negative) results.
Publication bias is particularly problematic when pooled analyses are performed as it leads to an overestimation of the effect size [2, 3]. Therefore, the issue is considered to be a problem primarily in those areas of research where meta-analyses are commonly performed [4–6]. Several studies on the determinants of publication have included basic science research abstracts [7–11], but so far only one study has looked into the occurrence of publication bias in this type of research .
There are several methods to detect publication bias in the medical literature. Most take advantage of the fact that small studies are more susceptible to publication bias and may, consequently, show larger treatment effects [13, 14]. One of the earliest reports of publication bias in the biomedical literature came from the area of hepato-gastroenterology. In 1965, T.C. Chalmers et al. examined possible reasons for the high variability in reported case fatality rates in post transfusion hepatitis and found an inverse correlation between sample size and fatality rate . More recently, Shaheen et al. were able to demonstrate publication bias in the reporting of cancer risk in Barrett's esophagus by using a funnel plot, a graphical method to detect publication bias . Preferential publication of studies with positive results was also shown following the presentation of abstracts in endoscopy research .
Publication bias can arise during several phases of the publication process . Abstracts submitted to scientific meetings are at an intermediate stage in the dissemination of research findings. The Digestive Diseases Week (DDW) is an important annual event in gastroenterology. The meeting is jointly organized by the American Gastroenterological Association (AGA), the American Association for the Study of Liver Diseases (AASLD), the American Society for Gastrointestinal Endoscopy (ASGE), and the Surgical Society for the Alimentary Tract (SSAT). Every year, several thousand abstracts are submitted to this meeting from investigators worldwide. Approximately half of these abstracts will eventually be followed by full articles in peer reviewed journals . We wished to study the determinants of publication in gastroenterological research using DDW abstracts as a representative sample. We were particularly interested to know whether there is any indication for publication bias, where this arises and whether it varies by type of research or national origin. We also wished to survey the abstract authors to determine their perception as to what factors determined if an abstract went on to a full publication.
Study design and sampling
This retrospective cohort study was based on abstracts submitted to DDW between 1992 to 1995 (n = 17,205). Each was screened for abstract acceptance status (presentation at a meeting), number of centers participating, country of origin and research type. The mode of presentation (poster vs. oral) was not known. Multi-center status was assumed if abstract authors from > 3 different centers were named. The country of origin was based on the affiliation of the first author. Research type was defined as basic science studies (BSS) if the unit of analysis was not an intact human. Controlled clinical trials (CCT) were prospective studies on the effects of diagnostic tests or therapeutic interventions, using parallel or cross over controls. "Other clinical research" (OCR) comprised human physiology experiments, epidemiological studies and uncontrolled therapeutic studies. The screening was performed by student raters following a short introduction into research methodology. Following the screening, 50 BSS, 100 OCR and 100 CCT abstracts were randomly selected from each year using computer assigned random numbers, for a total sample size of 1,000
Each abstract in the sample was reviewed in detail as to study design, sample size and statistical significance of the study results. Completeness of reporting and details of the study design were rated and combined into a summary score of formal abstract quality. The evaluation of abstracts was performed by raters with training in epidemiology and gastroenterology, masked in terms of authors, origin, acceptance for presentation at the meeting, and publication status. For all analytical studies (i.e. studies using designs appropriate for hypothesis testing), results were considered positive if statistical significance defined by a p – value < 0.05, or by a 95% confidence interval excluding unity was achieved for the main outcome of interest, or for the majority of outcomes if these were multiple [5, 20]. Results were considered negative if statistical significance was not achieved for the main outcome, or for the majority of outcomes. Equivocal results were defined if there were no statements concerning the statistical significance of the main outcome, or the majority of outcomes. Descriptive studies (case reports, case series, incidence/prevalence studies, qualitative research and descriptions of procedures) were excluded from the main analyses.
Abstract quality assessment tool
We developed an instrument to evaluate formal abstract quality based on previously validated quality scoring instruments for full papers [21, 22]. We also incorporated into the instrument published guidelines for the preparation of structured abstracts and short reports [23–25]. Depending on the study design, there were up to 21 items, including, for example, the definition of a research objective, use of randomization, control for confounding, use and reporting of adequate statistical methods and appropriateness of sample size. A summary score ranging from 0 – 1 could be calculated, adjusting for variations in the number of applicable items. The development and validation of this instrument are described in more detail in a separate article (Timmer, et al. Manuscript submitted for publication).
Data base searches
Publication status was assessed by searching MEDLINE, EMBASE, the Cochrane Library (CCT only) and Bios (BSS only) on the names of first and last authors. Only full manuscripts in peer reviewed journals as defined by Last  were considered to represent publications. The 1995 scientific impact factor (SIF) of a journal was used as an approximation of the prestige of a journal. This is a measure of the frequency with which the "average article" in a journal has been cited in a particular year or period .
Prior to the examination of publication rates, all continuous data were dichotomized. Sample size medians were calculated separately for different subgroups (CCT: cross over, parallel; OCR: physiology, epidemiology, therapy & diagnostics; BSS: animal studies. More details are presented in a separate publication ). Subgroup specific medians were used as cut-off values for high sample size. A high quality score was defined as a summary score > 0.63 which corresponds to the upper tertile in this sample. Similarly, high journal impact was defined by an SIF > 3. Abstracts were categorized into four country groups based on geopolitical regions: 1) USA/Canada, 2) North and West Europe (NW Europe), 3) South and East Europe (SE Europe), 4) other. France and Germany were classified as NW European, while SE Europe also included Mediterranean countries such as Israel and Turkey. Due to the low number of submissions from Australia, these were combined with NW-European abstracts. Analyses were re-run excluding Australian abstracts to examine whether this altered the results.
The major analyses were based on the information derived from the abstract evaluation and the data base searches. Annual publication rates were calculated using actuarial survival methods . Adjusted hazard ratios (HR) for the determinants of publication were calculated using Cox regression, or by multiple logistic regression if the assumption of proportional hazards was not met . The independent variables included in the initial model were: research type, region of origin, sample size category, quality score category, statistical significance, country group and multi-center status. The model was reduced using manual back and forth procedures, with variable selection based on the stability of the effect sizes, and on the statistical significance of the estimates (p < 0.1).
The association between abstract acceptance and subsequent publication was examined separately as the presentation of an abstract was not considered an independent variable but rather part of the pathway to full publication (an 'interim outcome'). Logistic regression was used to predict early publication (< 2 years after the meeting) and high impact publication within the subgroup of published studies.
Sample size considerations
The sample size was chosen to provide sufficient power (>80%) to detect a 20% difference in publication rates between studies with statistically non significant ("negative") results vs. studies with positive outcome. The calculation allowed for separate analyses for CCT and OCR, assuming a negative: positive outcome ratio of 1:2, and a 60% publication rate for positive outcome studies (= 0.05). Since at the beginning of this study, there were no data on publication bias following BSS abstract presentations and because of uncertainty about the applicability of the descriptors used, a higher relevant difference was chosen for this subgroup (30%) and a smaller sample size was possible. Statistical significance was assumed on a 95% confidence level (two tailed).
A four-page questionnaire was sent to authors of all abstracts, followed, if necessary, by up to two written reminders. The final reminder included a short version of the original questionnaire. The number of publications identified by the author survey were compared to those identified by the data base searches to calculate data base retrieval rates. Responder bias was assessed by comparing the characteristics of abstracts for which questionnaires were received with those for which no questionnaires were received. We corrected for responder bias and incomplete retrieval by multiplying the data base derived rate for the full sample by the reciprocal of the retrieval rate for the abstracts covered in the survey. Agreement between abstract and survey information was assessed using Cohen's kappa (κ) .
Models based on the mail survey included, in addition to those described above, the following: mode of abstract presentation, governmental funding, funding by industry, number of previous publications of the principal investigator and age and sex of the investigator.
Abstract characteristics and publication rates
Baseline characteristics of abstracts
CCT N (%)
OCR N (%)
BSS N (%)
descriptive studies (excluded)
total in analysis
acceptance of abstract
country region of origin
number of study centers
Number of publications and crude publication rates by research type
CCT N (%)
OCR N (%)
BSS N (%)
total N (%)
total in analysis
> 3 centers
Predictors of publication
data base based OR (95% CI)
survey based OR (95% CI)
number of applicable abstracts
abstracts with missing values
total in analysis
previous publications of PI
less than 10
more than 10
> 3 centers
Time to publication and impact factor of the publications
Annual publication rates were highest in the second year after submission. Of the identified publications, 67% were published within two years (median time to publication: 18 months). If a project was not published within four years, the chances for subsequent publication were only 3% per year.
Results of the mail survey
Questionnaires were received for 593 of 1,000 abstracts (46 returned undeliverable, response rate 62.2%). Of these, 58 were in short form. Response rates were higher for authors of subsequently published abstracts (69.9% vs. 55.6%, based on data base searches, p < 0.001), indicating significant responder bias. The survey identified 387 publications including 306 identified by the data base searches (data base retrieval rate 79%). The retrieval rate was lower for manuscripts published in languages other than English (33%; English language manuscripts: 80%, p = 0.03), however, this group was very small (n = 6). No other factors were associated with retrieval rates. The combined effect of incomplete retrieval by the data base searches on the one hand and responder bias in the survey on the other hand resulted in a significantly higher publication rate for the survey based analysis as compared to the data base search (64% vs. 47%). The publication rate corrected for these biases was estimated to be 58% (95% CI 55% to 61%).
Where survey information could be validated against information from the abstracts or from Medline, agreement was poor (abstract acceptance = 0.5, randomization (CCT only) = 0.2, statistical significance = 0.3). The poor agreement for statistical significance was due to the large proportion of abstracts with equivocally reported results. The majority of these (64%) had become statistically significant in the survey (8% negative, 27% equivocal). The date of publication was missing or incorrect in 58% of the reported publications.
Survey information and survey based publication rates
CCT N (%)
OCR N (%)
BSS N (%)
descriptive studies (excluded)
total in analysis
presentation at the meeting
statistical significance of results
age of principal investigator
< 35 years
> 50 years
sex of principal investigator
number of previous publications
up to 10
Reasons for non publication
Reasons for non completion or non submission
most important factor
lack of time
lack of interest
sample size/recruitment problems
limitations in methodology
publication not the aim
side effects/ethical problems
single decisive factor not reported
This study examine determinants of publication of gastrointestinal research, based on a large sample of abstracts submitted to the major scientific meeting in this field. The evaluation of the randomly selected abstracts showed a wide variety in terms of research topic, origin, quality and research type. Until 1999 all DDW abstracts were published as a supplement to Gastroenterology. Abstracts are often not included in reviews nor are they always easily available to the interested reader. The reliability of information from abstracts has been questioned [32–34]. It is important to identify the factors which influence full publication as selective underreporting of research may result in seriously distorted information .
Based on data base searches, approximately about half of the abstracts were subsequently published as full manuscripts. The proportion ranged from 42% for other clinical research to 52% for controlled clinical trials. This is in accordance with data reported for other meetings. Publication rates following abstract presentation have been published for a large variety of biomedical specialties. In a recent Cochrane review on publication rates following abstract presentation, 46 studies were identified, following up a total of 15 985 abstracts . Publication rates varied from 10% to 78%, with a median rate of 47%. This meta-analysis constitutes an update and extension to a previous study by Scherer, combining 2391 meeting abstracts from 10 studies, resulting in a pooled publication rate of 51% . Since the publication of the Cochrane review, more follow up studies were published, including two in subspecialties of gastroenterology [17, 38], two more studies following up gastroenterological abstracts are available in abstract form [19, 39]. The similarity in the reporting of publication rates across different meetings is striking. Only in the report by Eloubeidi, a significantly lower rate was reported (25%) . We suggest that this may be due to the restriction to endoscopy research which may not be representative of more general meetings.
The most important factor found to be associated with full publication in our study was the acceptance of the abstract for presentation at the meeting. We found an odds ratio of 2.3 (95% CI 1.7 to 3.1) for publication of accepted abstracts, as compared to rejected abstracts. This translates into a relative risk of 1.6 (95% CI 1.4 to 1.9), a number which is consistent with data reported by other investigators [8, 39–44]. Scherer calculated a pooled relative risk of 1.8 (95% 1.7 to 2.0) for publication, based on 7 reports . Generally, the determinants of abstract acceptance resemble those of subsequent publication, although additional factors may play a role [42, 45]. Acceptance of an abstract for presentation may influence the decision of authors to pursue full publication. We did not consider abstract acceptance an independent factor in the publication process, but rather a variable on the causal chain. Therefore, we did not include this factor in our multivariate analysis. Our findings on the determinants of abstract acceptance are reported in more detail in a separate publication .
Most previous studies focused on clinical research, or were restricted to randomized clinical trials only. Few data are available on publication processes for basic science. Generally, due to the higher heterogeneity in terms of study design and study objectives, results in this group are more difficult to interpret. However, most trends were similar in the subanalyses. Basic science research was found to take less time to publication as compared to clinical research. This may explain the higher publication rate for BSS as compared to clinical research which was described by Kiroff in a study involving a shorter follow up period, and was also confirmed in the meta-analysis by Scherer [12, 36]. In our study, the rate and the scientific impact of publication as measured by the scientific journal impact factor was similar between basic science research and controlled clinical trials.
A novel aspect was the international context which allowed for the examination of differences in publication rates based on the origin of the research. In a parallel study, North American origin was shown to be the most important predictor of abstract acceptance for presentation at the US-based DDW-meetings . In contrast, no difference in the proportion of published studies, nor in the impact of the publications was found between research from North America or NW Europe and Australia. There was some indication that abstracts from other countries than North America, NW Europe and Australia were of somewhat lower quality, resulting in a lower proportion of high impact publications and lower publication rates, however, the number of submitted abstracts from these other countries was low, and estimates were imprecise.
Ideally, the decision to submit or accept a manuscript for publication is based on the scientific quality of the research . This variable is particularly difficult to examine based on the limited information available from meeting abstracts. In a study on the fate of emergency medicine abstracts, abstract quality was assessed by a modification of Chalmers' rating system for the methodological quality of randomized clinical trials, as well as by a scientific originality rating . Originality was found to be associated with subsequent publication, but, as in a previous study using a similar modification, the methodology score was not predictive [6, 46]. However, only few design features were included in this score, and the instrument may not have been very sensitive to differences in formal quality. In contrast, we used a newly developed 21 item score, encompassing various aspects of completeness of reporting and design features to assess abstract quality. Due to the wide range of research types and topics within the sample, originality of the research could not be uniformly assessed. The positive association found for formal quality and prestige of the publication, however, may be taken as indicative of a good correlation between form and content, at least for the subgroup of controlled clinical trials, as was suspected by other authors .
A positive outcome of the study results was not significantly associated with subsequent publication. This is somewhat surprising in view of the consistent findings in the literature that show an about twofold chance for publication if the results are statistically significant . One reason may be the high proportion of equivocal results. The inability to correctly classify these abstracts is likely to have diluted the strength of the association studied. Nevertheless, publication bias was still evident from the higher impact of publications with positive results, which is is in accordance with the observation of a higher susceptibility for publication bias in higher impact journals [4, 6]. In addition, for basic science studies in particular, the low proportion of positive outcome studies indicated a biased abstract submission, a phenomenon also demonstrated by Callaham .
Limitations of this study include the low response rate in the author survey, which compromised the validity of an analysis based on author information. As significant responder bias and limitations to the reliability of the information given by the surveyed authors were evident, the results from the survey based models have to be interpreted with caution. On the other hand, the complementary use of data collection was important to gauge the reliablity of the information used.
Another limitation stems from the heterogeneity of the research projects and the international context. While this added valuable information on publication processes in a wider perspective, a few variables were more difficult to assess. Lack of comparability between study types may, for example, explain the failure to show an association between sample size and full publication. Similarly, national funding policies are likely to differ considerably, so that potential effects of source of funding on publication rates were possibly missed.
Based on the results of our study, there is subtle evidence for publication bias in gastroenterological research, expressed by a low proportion of negative outcome abstract submissions, especially in basic science, and by a lower proportion of high impact publications if the results are statistically negative. In the majority of cases, authors are responsible for the failure to publish. The acceptance of an abstract for presentation at a scientific meeting seems to be an important factor in the decision to submit a manuscript for publication. The submission of studies with statistically negative results needs to be encouraged, especially in basic science.
basic science studies
North and West
controlled clinical trial
other clinical research
Digestive Diseases Week
South and East
scientific impact factor
The authors wish to thank K. Dickersin, Providence, for expert advice and thoughtful comments on the manuscript. Thanks also to M. Dorgan, Edmonton, for assistance with the data base searches. A. Timmer was a fellow of the German Academic Exchange Service (DAAD). The study was supported by a grant from the Calgary Regional Health Authority R&D and by Searle Canada.
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