Selective reporting of outcomes within published studies based on the
nature or direction of their results has been widely suspected, but direct
evidence of such bias is currently limited to case reports.
To study empirically the extent and nature of outcome reporting bias
in a cohort of randomized trials.
Cohort study using protocols and published reports of randomized trials
approved by the Scientific-Ethical Committees for Copenhagen and Frederiksberg,
Denmark, in 1994-1995. The number and characteristics of reported and unreported
trial outcomes were recorded from protocols, journal articles, and a survey
of trialists. An outcome was considered incompletely reported if insufficient
data were presented in the published articles for meta-analysis. Odds ratios
relating the completeness of outcome reporting to statistical significance
were calculated for each trial and then pooled to provide an overall estimate
of bias. Protocols and published articles were also compared to identify discrepancies
in primary outcomes.
Main Outcome Measures
Completeness of reporting of efficacy and harm outcomes and of statistically
significant vs nonsignificant outcomes; consistency between primary outcomes
defined in the most recent protocols and those defined in published articles.
One hundred two trials with 122 published journal articles and 3736
outcomes were identified. Overall, 50% of efficacy and 65% of harm outcomes
per trial were incompletely reported. Statistically significant outcomes had
a higher odds of being fully reported compared with nonsignificant outcomes
for both efficacy (pooled odds ratio, 2.4; 95% confidence interval [CI], 1.4-4.0)
and harm (pooled odds ratio, 4.7; 95% CI, 1.8-12.0) data. In comparing published
articles with protocols, 62% of trials had at least 1 primary outcome that
was changed, introduced, or omitted. Eighty-six percent of survey responders
(42/49) denied the existence of unreported outcomes despite clear evidence
to the contrary.
The reporting of trial outcomes is not only frequently incomplete but
also biased and inconsistent with protocols. Published articles, as well as
reviews that incorporate them, may therefore be unreliable and overestimate
the benefits of an intervention. To ensure transparency, planned trials should
be registered and protocols should be made publicly available prior to trial