What Are Inclusion and Exclusion Criteria?
If you want to define which parts of members of the population should be included in the study, use inclusion and exclusion criteria. These two types of criteria are also known as eligibility criteria, and you have to be accurate while determining them for your research design, especially if it refers to clinical studies in medical science.
The latter requires entirely homogeneous groups (e.g., people with heart disease) to properly identify their needs. Let’s consider the characteristics of inclusion and exclusion criteria:
- demographic characteristics, including ethnicity, age, occupation, gender identity, and social status
- study-specific variables, such as the previous history of medical treatment, chronic conditions, type of disease and its stage of development, the technology used in the study, participants’ availability for follow-up research
- control variables, involving alcohol or tobacco use, additional medications, fitness levels
You will not be able to receive reliable results from your study without a proper definition of inclusion and exclusion criteria. It happens because you will never be confident about the existence of causal relationships between treatment and control groups. So the internal validity and generalizability (or external validity) of the obtained results will be distorted.
What Do the Inclusion Criteria Involve?
The inclusion criteria can define what features or attributes the potential research participants need to have to be included in the project. The most widely applied inclusion criteria are geographic, clinical, or demographic ones.
You are planning a clinical trial of a new medication meant to relieve insomnia. You need the following inclusion criteria here:
- ✔️ 35-65 years of age
- ✔️ chronic insomnia symptoms lasting for at least two months
- ✔️ previous therapy with one of the commonly used preparations
- ✔️ participants’ desire to continue the follow-up trials
If potential participants meet all these inclusion criteria, they can be chosen for your sample.
How to Define the Exclusion Criteria?
If you want to narrow down the sample by excluding some potential participants, you need to use exclusion criteria. They can also define those participants who were initially included in the research project.
Therefore, some participants that display specific characteristics that can interfere with the final study results, even if they meet all the inclusion criteria, can be withdrawn from the research with the help of the exclusion criteria.
Let’s look at typical exclusion criteria:
- ethical considerations, for instance, when a person is not able to provide consent to participate in the study or is a minor whose consent does not count without the adult’s agreement
- practical considerations, for instance, when the person is inaccessible at needed times, cannot read the survey questions, or is not located at the required community
In medical research, exclusion criteria play a great role. A person cannot participate in the research if they have one more medical disorder or are pregnant. For example, the following exclusion criteria can be applied to medical research on kidney disease:
- ✔️ the patient with the acute problem is pregnant
- ✔️ the patient has recently experienced the kidney surgery
- ✔️ the patient is ill with Type II diabetes
- ✔️ the patient cannot or refuses to provide informed consent
- ✔️ the patient has recently been on the dialysis
Potential participants of the research who meet one or several exclusion criteria have to be disqualified. They cannot participate in your research even if they meet all the inclusion criteria.
Inclusion and Exclusion Criteria: Other Examples and Explanations
As a researcher, you need to clearly define the inclusion and exclusion criteria before the start of your research project or trial.
Let’s consider the examples with bad and good solutions. Suppose you are researching the effect of exercise on curing insomnia in women with menopause syndrome.
Phrasing the inclusion criteria is important. The bad solution is “Subjects with insomnia should be included in the research.” The formulation is too extended and unclear because it is impossible to define whether the participants have insomnia only based on these criteria. That is why the description needs more details, such as “Subjects can be included in the study if they are women at the age of 40-55, were diagnosed with insomnia by their therapists and have experienced such symptoms as problems with falling asleep and staying asleep all night long at least 2 nights per week for a minimum of 2 months.” Here, you clarify the diagnosis and symptoms with the distinct time frames when the condition is continuously displayed.
Now, let’s tackle the exclusion criteria. The badly formulated criteria sound like “Subjects are excluded from the research if they intake anti-insomnia medications.” The definition of the criterion is too broad - there are many medications that can help against insomnia but do not influence the results of your study. They can be antidepressants, painkillers, or even heart-supporting additions.
The unclear definition may make the recruitment of the sample too difficult, and the validity of your study will suffer from it. You need to formulate the criterion more clearly - “Subjects are excluded from the study if they are intaking any medication that affects sleep quality right now, any prescription drugs or supplements that, as experts consider, may affect the results of the research.” Such a statement is more detailed, and it allows to control of extraneous variables that can impact the final data.
Therefore, you need to develop and review inclusion and exclusion criteria and even discuss them with potential participants to ensure they will be eligible.
Importance of Inclusion and Exclusion Criteria
If you research the characteristic features of a concrete subgroup of the population, clear definitions of inclusion and exclusion criteria are essential in any type of research design. You will be able to choose reliable and consistent samples in correspondence with all the objective factors. Due to these criteria, you will get a sample of participants who can accurately answer your research question.
If you deal with a clinical trial, inclusion and exclusion criteria are still more important because you may do harm to those participants (like pregnant women) with your intrusions or ethically exploit individuals who cannot comprehend the outcomes of their participation in the research. Any human-based study should consider these criteria for ethical reasons.
Remember!
You need to know how to justify your choice of all the inclusion and exclusion criteria. You cannot use exclusion criteria that make the sample too unified unless you mean to exclude the entire subgroup. The population you are researching has to be diverse and possess multiple characteristics or your study results will not be valid.
You also need to consider all the risks and advantages associated with your study. For example, if you are studying insomnia in women with menopause, you will not be able to justify the exclusion of Latino women. However, the exclusion of women under 40 can be justified because they do not experience menopause.
The main objective of any clinical trial is to support the hypothesis that the medication or treatment is effective and safe when used by those its meant for.
That is why it is important to know that all the participants are representative of the population you are studying to get increased external validity. Accordingly, when you formulate the clear inclusion and exclusion criteria, you support the claim that causal relationships can be observed between treatment and control groups. It will enhance the internal validity.
Other researchers will as well benefit from the clear formulation of inclusion and exclusion criteria. They will continue your research and may need to repeat certain procedures. Selecting the participants correctly will be vital for them because they will be successful in their replication studies.
Some fields of study, such as, for example, ethnographies, do not specify exclusion criteria. However, clear inclusion criteria help researchers in this field to define the population and samples easily and consistently. For example, if they study the use of bicycles in a specific location, they pick out the individuals who normally do that, and the overall objectives of the research will be met.
Final Thoughts
You can see now what inclusion and exclusion criteria are and what role they play in the research design. It is important not to underestimate their importance because the lack of accurate, valid, and consistent research criteria used for sample recruitment will lead to missing, false, or incomplete data. You will not be able to make valid conclusions based on such data, so your research’s external validity will be poor too. No other researchers will be able to replicate your study and continue it if it is invalid or biased. That is why you need to formulate inclusion and exclusion criteria beforehand.