This study assessed the prevalence of reporting statistically significant effect estimates in leading oral health journals and examined associations between the direction of the results and record characteristics. In the study timeframe, 82.4% full-text records reported a statistically significant effect estimate confirming the apparent preponderance towards publishing positive results within the oral health literature. The odds of publishing significant effect estimate were also associated with the continent of the corresponding author, the record type and registration of the record. This result is comparable to previous investigations which reported the reporting of significant results in dental speciality journals to range between 47 and 86%  and 75–90% . However, the same trend is not evident in the publishing of dental abstracts where the significance of the results does not predict the likelihood of publication . Although, the current study was conducted after a significant time lag between previous investigation [13, 14], it appears the publishing of records with significant effect estimates still dominates and may have increased over this time period. Indeed, investigations within medical speciality journals have found a highly significant trend of reporting positive results increasing on a yearly basis by 6% .
There was variation in the percentage reporting of significant effect estimates between the leading oral health journals included in this study. In addition, all speciality journals compared to general oral health types were less likely to publish positive results. As previously postulated, this could be a reflection of differences in the study types published in each journal . Articles published by authors based in Asia or other were more likely to publish records with positive findings compared to those based in Europe which mirrors the findings of previous studies [12, 13, 16]. This maybe reflective of the fact that trials carried out in developing countries are reported to show more positive findings compared to trials performed in developed countries . Registration of trials is encouraged to improve transparency in the conduct of the study but also to eliminate publication and selective reporting bias [18, 19]. Interestingly, registration of records was less likely to be associated with the reporting of significant effect estimates when compared to non-registered records which may suggest that registration is having the desired impact. This is also supported by the finding that at the study level, interventional type studies which are encouraged to be registered are less likely of reporting positive findings compared to in-vitro studies. This corroborates the findings of a similar studies . This really highlights the importance of correct interpretation of studies with a perceived weaker design as they are more likely to report exaggerated treatment effects whereas interventional studies such as Randomised Clinical Trials can contradict the findings reported by observational studies [20, 21].
The reproducibility of research study design is reported to be poor . Conversely, if replication of study design can be achieved, the results of such studies are more likely to contradict the reported initial stronger results over time, independent of the study design . If an improvement in research methodology is excluded, then reporting of a significant result could be dependent on other factors . First of all, it could be the fact that hypotheses tested are true. However, this needs to be balanced with the fact that authors may be confirming known hypothesis in order to get “publishable’ results . Authors who detect non-significant results decline to submit for publication or these results are turned into a positive direction through post-hoc analyses, selective reporting and reinterpretation . The consequences of selective reporting or “p-hacking” where investigators carry out multiple statistical tests and then report only those which produce significant results has been highlighted .
Although every attempt was made to elicit the primary outcome from each record, when it was not obvious, the first outcome was analysed which introduces a degree of subjectivity and potential interpretation of misleading outcome results. The presence SPIN, where beneficial effects of an intervention are highlighted despite a non-significant difference detected between treatment interventions has been established in dental speciality trials [25,26,27]. Indeed, SPIN related to the focusing on significant within-group comparisons, focusing on a significant primary outcome when there are several co-primary outcomes and focusing on significant secondary outcomes has been reported in orthodontic trial abstracts . To avoid any subjectivity, future assessments could review the registration record, published protocol, or duplicate publication, when the primary outcome is not specifically reported.
The selected in-vitro records also include animal studies and the only articles excluded were case reports, review articles, editorials and systematic reviews. The aim of our study was to assess the prevalence of reporting significance results in oral health journals and to see if this known problem still persists. The justification for the inclusion of in-vitro records is that they include an experiment, record an outcome and commonly include the results of statistical tests. Statistically significant effect estimates in in-vitro studies can influence decisions in conducting other similar or higher-level studies. In the analysis, it is also interesting to see that statistically significant records are more prevalent in in-vitro records compared to clinical studies. For example, there is evidence that RCTs which are usually the most rigorous studies have the lowest prevalence of significant results which may imply that in vitro studies may pass under the radar and thus more attention should be paid when interpreting the in vitro studies results apart from whether the results are generalizable due to the in-vitro setting.
Reasons for non-publication of records despite the strength of the findings include lack of time, incomplete study status, low priority and issues with co-authors . However, researcher related factors are primarily cited [15, 28]. In the investigation of publication of abstracts following presentation at a biomedical conference, the most frequently cited reason by authors to not publish was a lack of time . Methods to encourage publishing of non-significant findings could be suggested at the study ethical approval stage. For instance, ethics committees could suggest the reporting of the results of clinical studies regardless of the direction of the effect . Mandatory registration of clinical trials, enforcing guidelines for accurate reporting and creating journals of negative results have also been suggested [4, 31,32,33,34]. Regarding the latter, such journals could be funded by public or charitable support . Furthermore, trial funding agencies could make it a pre-requisite to publish both significant and non-significant findings of primary hypotheses tested . The disclosure of funders or reporting of funding sources was not collected as a variable in this study. Future studies, should consider this characteristic and its relationship on the publication of studies with positive results, as trials with a high or unclear risk of sponsorship bias are reported to be associated with larger treatment effect size estimates .