Guideline for Selecting Instruments for a Core Outcome Set
Improve the selection of outcome measures when developing a COS with our systematic and consensus-based guidance.
A joint initiative between COSMIN and Core Outcome Measures in Effectiveness Trials (COMET) aimed to develop a guideline on how to select outcome measurement instruments for outcomes included in a COS.
What is a Core Outcome Set (COS)?
A COS is a consensus-based agreed minimum set of outcomes that should be measured and reported in all clinical trials of a specific disease or trial population; it is a recommendation of what should be measured and reported in all clinical trials. An outcome refers to what is being measured. It is also referred to as a construct or domain. In the context of a clinical trial it refers to what is being measured on trial participants to examine the effect of exposure to a health intervention.
Once the COS is defined, it is then important to achieve consensus on how these outcomes should be measured, i.e. which outcome measurement instruments should be selected. An outcome measurement instrument refers to how the outcome is being measured. It is a tool to measure a quality or quantity of the outcome. The tool can be a single question, a questionnaire, a score obtained through physical examination, a laboratory measurement, a score obtained through observation of an image, etcetera.
Preferred approach for selecting outcome measurement instruments in four steps
The guideline consists of four main steps to be taken in the selection of outcome measures for outcomes included in a COS:
Aspects to consider before starting to search for outcome measurement instruments:
- The construct (i.e. outcome or domain) to be measured
- The target population (e.g. age, gender, disease characteristics)
Finding existing outcome measurement instruments
COS developers should aim for finding all existing outcome measurement instruments
When finding instruments, COS developers can have three sources of information: 1) systematic reviews, 2) a literature search, and 3) other sources (optional):
- COS developers use existing, good quality, and up-to-date systematic reviews of outcome measurement instruments
- Medline (e.g., through the PubMed or OVID interface) is considered the minimum database to consult in finding all existing outcome measurement instruments. An additional search in EMBASE is highly recommended.
- b. Reference lists of the included studies should be checked to find all existing outcome measurement instruments
Additional sources may be considered as optional sources in finding relevant outcome measurement instruments
Quality assessment of outcome measurement instruments
To evaluate the quality of the outcome measurement instruments COS developers evaluate 1) the measurement properties and 2) the feasibility aspects of the identified instruments:
- Evidence on the measurement properties should be available in the target population
- Feasibility aspects should be taken into consideration in the selection of outcome measurement instruments for a COS
Generic recommendations on the selection of outcome measurement instruments for a COS
- Select only one outcome measurement instrument for each outcome (e.g. construct or domain) in a COS
- The minimum requirements for including an outcome measurement instrument in a COS are: at least strong3 evidence for good4 content validity and for good4 internal consistency (if applicable), and if the instrument is feasible
- A consensus procedure to agree on the instruments for each outcome included in the COS should be performed among all relevant stakeholders, including patients
The process for the selection of outcome measurement instruments for core outcome sets is being displayed in a figure.
Figure 1. Flowchart for the selection of outcome measurement instruments for core outcome sets