Inclusion & exclusion criteria explained

At ICORD, clinical and behavioural research studies are conducted that involve human volunteers (participants) to expand knowledge and help us gather new information on a specific question or field of study. You may have noticed that some studies have eligibility criteria called inclusion criteria and exclusion criteria. Inclusion criteria are the factors that allow a person to participate in a study, whereas exclusion criteria are the factors that disqualify a person from participating. There are many reasons why a person can or can’t participate in a study.

Eligibility criteria are created by researchers to ensure findings are applicable to the questions being asked as well as to minimize harm to participants. The purpose of the study informs the inclusion and exclusion criteria therefore the criteria can differ among research studies with different purposes. Choosing criteria is important because it ensures participants provide the necessary information to address research questions. A different but equally important reason for criteria selection is based on the safety of participants. If researchers have reason to believe an intervention or study may cause harm to a group of individuals, people with these characteristics will be excluded from their study. When researchers select criteria, they do their best to ensure the rigour and validity of study findings, while also maintaining participant safety. Therefore, one study may include participants who were excluded from another study. This does not indicate that the criteria are unethical but instead that the purposes of the studies are different.

Establishing inclusion and exclusion criteria for study participants is a required standard practice. ICORD studies involving human participants are approved by a Research Ethics Board (REB) before the researchers recruit participants. The REB checks that the selection of participants will be fair and equitable, while considering the purpose of the research study and the research setting.

Inclusion and exclusion criteria are deemed ethical based on Chapter 4 (Article 4.1) of the Tri-Council Policy Statement: Ethical Conduct for Research Involving Humans, which states:

Taking into account the scope and objectives of their research, researchers should be inclusive in selecting participants. Researchers shall not exclude individuals from the opportunity to participate in research on the basis of attributes such as culture, language, religion, race, disability, sexual orientation, ethnicity, linguistic proficiency, gender or age, unless there is a valid reason for the exclusion.

Inclusion criteria for ICORD studies might include:

  • having a documented spinal cord injury, possibly at a particular level or severity;
  • using a wheelchair to get around or sometimes walking;
  • being old enough to give informed consent to participate in the study.

Exclusion criteria for ICORD studies might include:

  • having other health conditions that could make taking part in the study unsafe;
  • being over or under a specific weight, height, or age.

Let’s look at some examples:

  • A research study looking at pregnancy after spinal cord injury might only recruit people with a uterus in a certain age range. The criteria could be justified based on biology of participants and the research purpose.
  • A research study looking at flaccid bowel after spinal cord injury might exclude people with injuries above T12 because we know from the literature that flaccid bowel is more common with injuries below T12.
  • A research study looking at exercise after spinal cord injury might exclude a person with a cervical spinal cord injury because of the risk for autonomic dysreflexia – a serious condition that can affect people with injuries at T6 and above.
  • A research study testing a robotic exoskeleton might only include participants within a certain height and weight range. This is not due to researchers discriminating against body size.
  • When equipment is still in a testing phase, the prototype machine can only fit people within a certain height and weight much like how a prototype shoe can only fit one shoe size.
    A research study looking at the experiences of individuals who use an app-based exercise program might only recruit people who have already downloaded and used that app. The criteria could be justified because individuals must have used the service to be able to comment on their experiences.
  • A research study exploring the experiences of individuals with SCI who identify as non-binary, transgender, or gender diverse in interactions with healthcare workers might exclude people who identify as cis-gendered. The criteria could be justified because past research indicates individuals who are gender diverse are more likely to face discrimination when seeking healthcare.

Inclusion and exclusion criteria are based on normative assumptions. Researchers need ranges because it is not always possible to test every person interested in a study for a specific exclusion criterion. From the examples above, not all people with cervical spinal cord injuries have autonomic dysreflexia. A person with a cervical level injury who does not experience autonomic dysreflexia may feel their exclusion is unfair, however, the criterion is based on safety harm reduction. Another example is age. Age ranges are common in inclusion and exclusion criteria and generally based on scientific evidence that core biological, sociological, and psychological factors change with advancing age. As researchers expand their findings thanks to the help of carefully chosen eligibility criteria, this newfound knowledge allows inclusion criteria to expand or change with subsequent studies.

For more information, please refer to the following resources: