A study published in JAMA Internal Medicine in February 2015 (which is unfortunately behind a pay wall) indicated that clinical trials on treatments for cardiovascular diseases that were registered were less likely to report positive results than unregistered trials. Researchers looked at clinical trials that were published on PubMed in December 2012 and found 191 on cardiovascular diseases, 45% of which had been registered. Of the trials that weren’t registered, 70% reported positive results compared to 52% of the trials that were registered. Publications from trials that weren’t registered also included fewer details about the trial’s methods.
In his Editor’s Note accompanying the article, Joseph Ross writes that, “The higher rate of reporting positive outcomes among non-registered trials raises concerns about bias, suggesting, but not proving, that clinical trial registration at least partially mitigates selective outcome reporting.”