Publication bias: evidence of delayed publication in a cohort study of clinical research projects.
OBJECTIVE
To determine the extent to which publication is influenced by study outcome.


METHODS
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.


METHODS
University hospital, Sydney, Australia.


METHODS
748 eligible studies submitted to Royal Prince Alfred Hospital Ethics Committee between 1979 and 1988.


METHODS
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 (P > or = 0.10) (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 < or = 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.
Stern
J M
JM
National Health and Medical Research Council Clinical Trials Centre, University of Sydney, NSW, Australia.
Simes
R J
RJ
D035843:Biomedical Research; D002986:Clinical Trials as Topic; D015331:Cohort Studies; D035081:Editorial Policies; D033181:Information Dissemination; D016016:Proportional Hazards Models; D017594:Publication Bias; D036301:Qualitative Research; D012106:Research; D012189:Retrospective Studies; D013997:Time Factors; D016896:Treatment Outcome