Limited public money is available for funding research and the majority of clinical research undertaken is funded by industry. Mechanisms to regulate conflicts of interest within the research process have been implemented. However, these policies by themselves do not protect against potential sponsorship bias that would affect research results to inform decision makers when using the results of these trials. Therefore, the main aim of this study was to evaluate the influence of sponsorship bias on the treatment effects of RCTs.

This was a meta-epidemiological study. A random sample of RCTs included in meta-analyses of physical therapy (PT) area were identified. Data extraction including assessments of appropriate influence of funders was conducted independently by two reviewers. To determine the association between biases related to sponsorship biases and effect sizes, a two-level analysis was conducted using a meta-meta-analytic approach.

We analysed 393 trials included in forty-three meta-analyses. The most common sources of sponsorship for this sample of PT trials were government (n = 205, 52.16 percent) followed by academic (n = 44, 11.2 percent), and industry (n = 39, 10 percent). The funding was not declared in a high percentage of the trials (n = 85, 22 percent). The influence of the trial sponsor was assessed as being appropriate in 246 trials (63 percent) and considered inappropriate/unclear in 147 (37 percent) of them. There was a significant difference in effects estimates between trials with appropriate and inappropriate influence of funders (ES= 0.15; 95% CI -0.03, 0.33;). Trials with inappropriate/unclear influence of funders tended to have on average a larger effect size than those with appropriate influence of funding

Treatment effect size estimates were 0.15 larger in trials with lack of appropriate influence of funders. Systematic reviewers should perform sensitivity analyses based on appropriateness of influence of sponsorship in included trials.