What is a Core Outcome Set


When researchers study healthcare treatments, they measure how well the treatments work using outcomes (such as symptom improvement or side effects). However, different studies on the same condition often measure different outcomes. This makes it hard to compare results and find the best treatments.

Sometimes, researchers also fail to report all the outcomes they originally planned to measure, which can lead to biased or incomplete information.


A Core Outcome Set (COS) is a standard list of key outcomes that should be measured in every study about a specific health condition. If all studies use the same outcomes, researchers can:

  • Compare and combine study results.
  • Reduce wasted research.
  • Ensure the most important outcomes are always measured and reported.

To create a Core Outcome Set, researchers, doctors, patients, and carers work together to decide which outcomes are most important. They use consensus methods such as surveys, discussions, and meetings to reach an agreement.


Patients and carers have first hand experience of living with a condition. They can highlight outcomes that matter to them but may be overlooked by doctors or researchers. Including their input ensures that research focuses on what truly impacts people’s lives.


Once agreed, Core Outcome Sets should be used in all future studies on that condition. Researchers can still measure extra outcomes if needed, but the core outcomes ensure that studies are consistent, complete, and useful for making treatment decisions.


The COMET (Core Outcome Measures in Effectiveness Trials) Initiative helps researchers develop and use Core Outcome Sets. It provides:

  • A database of existing and in-progress Core Outcome Sets.
  • Resources and support for researchers and patients involved in this work.