Systematic Review of the Empirical Evidence of Study Publication Bias and Outcome Reporting Bias — An Updated Review
OBJECTIVE To investigate the association between trial characteristics, findings, and publication. The major factor hypothesized to be associated with publication was "significant" results, which included both statistically significant results and results assessed by the investigators to be qualitatively significant, when statistical testing was not done. Other factors hypothesized to have a possible association with publication were funding institute, funding mechanism (grant versus contract versus intramural), multicenter status, use of comparison groups, large sample size, type of control (parallel versus nonparallel), use of randomization and masking, type of analysis (by treatment received versus by treatment assigned), and investigator sex and rank. DESIGN Follow-up, by 1988 interview with the principal investigator or surrogate, of all clinical trials funded by the National Institutes of Health (NIH) in 1979, to learn of trial results and publication status. POPULATION Two hundred ninety-three NIH trials, funded in 1979. MAIN OUTCOME MEASURE Publication of clinical trial results. RESULTS Of the 198 clinical trials completed by 1988, 93% had been published. Trials with "significant" results were more likely to be published than those showing "nonsignificant" results (adjusted odds ratio [OR] = 12.30; 95% confidence interval [CI], 2.54 to 60.00). No other factor was positively associated with publication. Most unpublished trials remained so because investigators thought the results were "not interesting" or they "did not have enough time" (42.8%). Metaanalysis using data from this and 3 similar studies provided a combined unadjusted OR of 2.88 (95% CI, 2.13 to 3.89) for the association between significant results and publication. CONCLUSIONS Even when the overall publication rate is high, such as for trials funded by the NIH, publication bias remains a significant problem. Given the importance of trials and their utility in evaluating medical treatments, especially within the context of metaanalysis, it is clear that we need more reliable systems for maintaining information about initiated studies. Trial registers represent such a system but must receive increased financial support to succeed.