A study published in PLOS ONE in the past week investigated if the number of null results in large National Heart Lung, and Blood Institute (NHLBI) funded trials has increased over time.

The authors identified all large NHLBI supported RCTs between 1970 and 2012 evaluating drugs or dietary supplements for the treatment or prevention of cardiovascular disease. In total, 17 of 30 studies (57%) published prior to 2000 showed a significant benefit of intervention on the primary outcome in comparison to only 2 among the 25 (8%) trials published after 2000 (p=0.0005). The authors also found that pre-registration in clinical trials.gov was strongly associated with the trend toward null findings.

In conclusion, the authors stated: “The number NHLBI trials reporting positive results declined after the year 2000. Prospective declaration of outcomes in RCTs, and the adoption of transparent reporting standards, as required by clinicaltrials.gov, may have contributed to the trend toward null findings.”.

A press release providing further commentary about this article can be found here. A blog from Nature on the article can be found here.

In the figure above, positive trials are indicated by the plus signs while trials showing harm are indicated by a diagonal line within a circle. Prior to 2000 when trials were not registered in clinical trials.gov, there was substantial variability in outcome. Following the imposition of the requirement that trials preregister in clinical trials.gov the relative risk on primary outcomes showed considerably less variability around 1.0.