Publication Bias and Research on Passive Smoking: Comparison of Published and Unpublished Studies

Context.— The results of reviews may be biased by delays in publication and failure to publish nonsignificant results. Objective.— To determine the extent of unpublished results on the health effects of passive smoking and whether passive smoking studies with statistically nonsignificant results would have longer time to publication than those with statistically significant results. Design.— Semistructured telephone interviews of principal investigators of published or unpublished studies funded between 1981 and 1995, identified by information obtained from 76 (85%) of 89 organizations contacted that potentially funded research on passive smoking. Participants.— Seventy-eight investigators were eligible and could be located; 65 (83%) responded. They had conducted 61 studies of the health effects of passive smoke in humans or animals between 1981 and 1995 that met the criteria for the analysis of time to publication. Main Outcome Measure.— Time to publication for published studies and statistical significance of results of published and unpublished studies. Results.— Fourteen of the 61 studies were unpublished. Median time to publication was 5 years (95% confidence interval [CI], 4-7 years) for statistically nonsignificant studies and 3 years (95% CI, 3-5 years) for statistically significant studies (P=.004). Statistically significant results (P=.004), experimental study design (P=.01), study size less than or equal to 500 (P=.01), and animals as subjects (P=.03) were predictive of time to publication. When the studies with human participants were analyzed separately, only statistically significant data were predictive of publication (P=.007). Multivariate analysis of all studies indicated that statistical significance (P=.001) and study design (P=.01) were the only independent predictors of time to publication, while for the human studies only statistical significance was predictive of publication (P=.007). Conclusion.— There is a publication delay for passive smoking studies with nonsignificant results compared with those with significant results.