Publication bias: evidence of delayed publication in a cohort study of clinical research projects

Abstract Objectives: To determine the extent to which publication is influenced by study outcome. Design: A cohort of studies submitted to a hospital ethics committee over 10 years were examined retrospectively by reviewing the protocols and by questionnaire. The primary method of analysis was Cox's proportional hazards model. Setting: University hospital, Sydney, Australia. Studies: 748 eligible studies submitted to Royal Prince Alfred Hospital Ethics Committee between 1979 and 1988. Main outcome measures: Time to publication. Results: Response to the questionnaire was received for 520 (70%) of the eligible studies. Of the 218 studies analysed with tests of significance, those with positive results (P<0.05) were much more likely to be published than those with negative results (P0.10) (hazard ratio 2.32 (95% confidence interval 1.47 to 3.66), P=0.0003), with a significantly shorter time to publication (median 4.8 v 8.0 years). This finding was even stronger for the group of 130 clinical trials (hazard ratio 3.13 (1.76 to 5.58), P=0.0001), with median times to publication of 4.7 and 8.0 years respectively. These results were not materially changed after adjusting for other significant predictors of publication. Studies with indefinite conclusions (0.05 P<0.10) tended to have an even lower publication rate and longer time to publication than studies with negative results (hazard ratio 0.39 (0.13 to 1.12), P=0.08). For the 103 studies in which outcome was rated qualitatively, there was no clear cut evidence of publication bias, although the number of studies in this group was not large. Conclusions: This study confirms the evidence of publication bias found in other studies and identifies delay in publication as an additional important factor. The study results support the need for prospective registration of trials to avoid publication bias and also support restricting the selection of trials to those started before a common date in undertaking systematic reviews. Key messages This retrospective cohort study of clinical research projects confirms the findings of publication bias found in previous studies Delay in the publication of studies with negative results has been identified as an additional important factor in publication bias With the recognised importance of evidence based medicine, these results have important implications for the selection of studies included in systematic reviews Prospective registration of clinical research projects will avoid many of the problems associated with publication bias However, it is also important to restrict inclusion in systematic reviews to studies started before a certain date to allow for the delay in completing studies with negative results