Worldwide clinical trials are the mechanism by which the safety and effectiveness of medicines and interventions are assessed. The effectiveness of interventions are assessed by outcomes such as decreased blood pressure or frequency of seizures, or changed weight. Harms of treatments are also relevant. Statisticians demonstrated mathematically that there was a systemic problem with outcome selection and reporting bias across the entire clinical trials sector. The impacts of taking multiple measurements and only reporting a selected subset of measurements, or selected subgroups of patients were modelled. This provided clear evidence of the need for change. This work led to the COMET Initiative (Core Outcome Measures in Effectiveness Trials), with important policy and societal consequences. COMET is now the international standard for harmonising outcomes for specific health conditions. Decisions about treatments are made more robust by ensuring that effectiveness data can be compared and combined. Trials have become more relevant to patients. COMET has transformed patient participation in outcome-setting from 16% up to 2013 to 93% in 2020. COMET is endorsed by mutliple healthcare decision-making organisations including guideline developers such as NICE, and trial funders including the NIHR and the pharmaceutical industry.