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How are core outcome sets developed?

When researchers design research studies to investigate treatments for health conditions they need to measure outcomes that are important and relevant to those people affected by the condition. To decide which outcomes are important researchers need to get everyone's opinion and try to reach an agreement, or “consensus”, on the most important outcomes.

In order to do this, researchers carry out a 'consensus exercise’. One way of doing this is by using something called a 'Delphi' study. In a Delphi study, researchers identify groups of people who are “experts” or stakeholders in the health condition they are interested in. “Stakeholders” are:


  • People with personal experience of the condition, for example, patients, carers and service users (it doesn’t matter how long the person has had the condition for, their opinion is incredibly valuable).

  • Health professionals with expertise in treating and caring for people with the condition.

  • Researchers, investigating the condition.


Stakeholders taking part in a Delphi study are asked to give their opinion on what outcomes are most important. The study is anonymous to make sure everyone has an equal say.


Early on in the study


The research team will have developed a long list of possible outcomes that they want to ask the stakeholders about. This has been created after looking at research papers. 


What happens next?


The long list is sent by email to all participants in the form of an electronic survey (Read here how to become a participant). All are asked to score the importance of each outcome on a scale from 0 (not important at all) to 9 (critical). If, in their opinion, there are key outcomes missing from the list, they are encouraged to add these to the list. We refer to this as “Round 1” of the Delphi study.


The research team then summarises the responses from the group as a whole and send this summary back to each participant in what we refer to as Round 2 of the Delphi process.


No one in the group can see another individual’s scores; they can only see the overall results for the group as a whole. Using this information each participant is asked to reflect on their own view and on the view of the group and to decide whether to stick with their original rating or change it.

Image by John T

Throughout the whole process, no one is under any pressure to change their rating if they don’t want to. It is perfectly fine for people to stick with their original rating even if they rated the outcome differently from the rest of the group.


The responses of the participants are then sent back to the research team who again collate the information.

Finally, in a third round, the research team will decide in a final meeting what outcomes will be included into the core outcome list

At the end of this process, the research team produce a report on what the experts have agreed as the most important outcomes. These are called the ‘core outcomes’ for a particular health condition.

This plain language summary was adapted from the COMET-initiative

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