At the end of a clinical trial, researchers assess the effects of the intervention on the health and survival of participants with a disease such as cancer. To do this, they use specific measures of how well the intervention worked to improve participants’ outcomes. One such measure is known as event-free survival, which is the length of time that participants remain free of complications and events after undergoing primary treatment for their disease. Researchers may also look at progression-free survival, which is the length of time that participants do not show signs of disease progression (do not get worse) during and after treatment. Another measure, called overall survival, is the length of time that participants are still alive following diagnosis or treatment.

Researchers can prioritize any of these measures — event-free survival, progression-free survival, overall survival, or another outcome measure — as the outcome that is the most important for evaluating how well an intervention worked. This is known as the primary outcome measure or primary endpoint, which is essentially the main finding of the clinical trial. Researchers often look at additional outcomes to provide supplemental information about the effects of an intervention and to help interpret the main finding. These are known as secondary outcome measures or secondary endpoints.