The need to standardise outcomes across research in the same area
Heterogeneity in how trials have been done can bring richness to systematic reviews and meta-analyses by testing different forms of an intervention, action or strategy and by testing these in different settings and with different populations. However, heterogeneity in the outcomes can hamper or prevent the ability of reviewers to pool the findings of studies in meta-analyses [5, 6] and will impede efforts to streamline the systematic reviewing process . Inconsistency in outcome measurement across trials in the same area, a lack of data on important outcomes from all trials, and selective reporting of outcomes compound the problems faced by users of research [8, 9]. One way to reduce this waste is through the development of agreed upon, standardised sets of outcomes for research in specific areas of health and social care. The implementation of these core outcome sets in all trials in a particular topic area would maximise the ability of the researchers themselves, reviewers and others to compare, contrast and combine the findings of the separate trials. Their use is now encouraged by funders of research, including the National Institute for Health Research in the UK .
Core outcome sets
If core outcome sets are to meet their potential to help researchers ensure that the outcomes they measure and report are the most relevant to users of their trial, these outcome sets need to be developed in a collaborative way with appropriate representation from key stakeholders. This is likely to include practitioners, patients and others users of health and social care services, as well as researchers, research funders, policy makers and those who pay for health and social care services. Although sporadic examples of what might now be considered to be core outcome sets have existed for some time , and well-established ones, such as OMERACT for rheumatoid arthritis, appear to have had an impact on trial design , the use of these sets is still in a period of relative infancy. Over the last five years, the COMET Initiative has been seeking to support others in the development and uptake of core outcome sets across health and social care  and has shown the wide variety of methods that have been used to develop them . Guidance is beginning to appear , and a recent systematic review identified nearly 200 examples .
Using core outcome sets when registering a trial
These core outcome sets have been brought together in a unique resource in the COMET database, which is freely available on the internet (www.cometinitiative.org/studies/search). This makes it much easier for researchers to identify a core outcome set if one already exists, as is evidenced by the fact that more than 25,000 records had to be checked as part of the systematic review . As part of the planning for their trial, researchers who wish to maximise the potential impact of their findings need to ensure that they will measure outcomes that will make it easier to show the relevance of those findings. They can do this by determining whether or not a core outcome exists, and in some cases, people embarking on a programme of research have developed a core outcome set to ensure the relevance of the outcomes they will measure and report . Researchers should consider a core outcome set when designing their trial. This does not restrict the outcomes that they will measure but does provide them with the minimum outcomes that they need to include. They remain able to include additional outcomes, especially those that may be particularly pertinent to their study.
Then, when it comes to registering a trial, the trial registry should encourage researchers to note their use of the core outcome set and to specify each of the outcomes from the core set, as well as any additional outcomes, that they will measure. Specifying the outcomes from the core outcome set and using the terms for these outcomes that were used in that core outcome set will facilitate searching by users of the registry. In fact, this could be made even easier if the registry entry could be automatically populated with the outcomes through a dynamic link to the core outcome set cited by the researchers. Another benefit from considering the core outcome set in the design and registration of their trial is that researchers may be helped in their choice of primary outcomes. These outcomes are likely to be used to determine the sample size for the trial but also will highlight the areas on which the researchers will focus. For example, the primary outcomes will be those on which they will focus most when seeking to minimise loss to follow-up and missing data.
When others inspect the registry entry for a closed trial, the explicit use of a core outcome set will facilitate one of the other benefits of a prospective registry: the ability to determine the risk of bias from measured, but not reported, outcomes. If outcomes from the core outcome set are not subsequently included in reports of the trial, this will help reviewers and others to investigate the potential impact of selective reporting bias on any decisions they wish to make.