the target population in research is

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Selecting the Study Participants

Bernadette capili , phd, np-c.

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(212) 327-8405, [email protected]

Defining the target population is an essential part of protocol development to ensure that the study participants are well suited to the research question ( Hulley et al., 2013 ). The target population is the entire group of people who share a common condition (disease process) or characteristic the researcher is interested in studying ( Elfil & Negada, 2017 ). While the ' accessible population' is the geographically and temporally classified subset that is available for the study participant recruitment ( Hulley et al., 2013 ). The clinical and demographic characteristics of the accessible population are determined using a ruled based eligibility criterion in which all study participants must have to join a study. The choice of study participants must also consider the cost in time and money to obtain the required number of study participants. This paper in the Step by Step Research series provides an introduction to designing the eligibility criteria and recruitment considerations to select the study participants. Future articles will discuss sampling (random and non-random) approaches and sample size determination.

Introduction to Designing the Eligibility Criteria

All clinical research protocols must have an Eligibility Criteria Section , which includes both an inclusion and exclusion criteria. Common elements used to determine suitability for a study are age, gender, medical condition, specific disease examined, and medications. The development of the study criteria must also consider participant safety and review potential interactions with an intervention such as a study medication or a study diet that might interfere with a participant's current medication or concurrent health condition. The eligibility criteria should be representative of the target population to improve understanding of the disease processes, new treatment options, and socio-demographic trends ( Hulley et al., 2013 ). To illustrate the development of the eligibility criteria, see table 1 : Designing the Eligibility Criteria: Clinical Trial for High Triglycerides Among Individuals with HIV Using Fish Oil Versus Placebo. Discussed below is the rationale for the selection of the inclusion and exclusion criteria for this study.

Designing the Eligibility Criteria: * Clinical Trial for High Triglycerides Among Individuals with HIV Using Fish Oil Versus Placebo

Adapted with permission:(Cummings, 2013)

( Capili & Anastasi, 2013 )

Inclusion Criteria Considerations

The inclusion criteria describe the study participants that fulfill the needs of the clinical research question. Commonly used elements for the inclusion criteria include demographic characteristics, clinical attributes, geographic factors, and temporal considerations ( Panacek & Thompson, 2007 ) ( Hulley et al., 2013 ). Defining the inclusion criteria helps to maintain a degree of homogeneity among the accessible population and narrows the available pool of eligible study participants that reflect the study question ( Panacek & Thompson, 2007 ). The inclusion criteria should also address the capacity of the participant to understand the study goals, the study requirements (i.e., attending study visits, completing questionnaires, blood draws, taking study medications), and the potential risks associated with joining a study.

For example, the study exploring the efficacy of over-the-counter fish oil supplementation among HIV+ individuals with high triglyceride considers an age limitation since fish oil can reduce the CD4+ level, and advanced ages are associated with the accelerated loss of the CD4+ level. A triglyceride parameter (fasting serum triglyceride levels between 150-500 mg/dl) is also required to capture participants with the condition, yet limits those with extremely elevated levels and likely eligible for prescription fish-oil treatment. Additionally, parameters for the CD4+ level (CD4+ T cell count ≥ 300 cells/mm 3 ) is necessary to ensure the participants' immune systems are stable. Travel to the study sight is another vital consideration. Geographic limitations are essential if virtual study visits (i.e., web-based interventions) are not possible, and face-to-face interactions are necessary. Using geographic boundaries may reduce missed study visits due to the distance of the study sight from participants' homes. An example of a temporal characteristic using the sample study is the requirement to wait for a minimum of six months or more after starting a new HIV drug regimen. The waiting period reduces the likelihood that the elevated triglyceride level was a temporary elevation and returns to normal levels once the proper dosage achieved.

The informed consent process is the final consideration for the inclusion criteria. A future article in this series will discuss the ethics, including concerns relating to the potential for coercion or undue influence, and the ability of the study participant to provide verbal and written consent.

Exclusion Criteria Considerations

The exclusion criteria contain the characteristics of the potential study participant who meet the inclusion criteria but have conditions that could affect the outcome of the study in terms of the success or the increased risks of possible adverse effects ( Patino & Ferreira, 2018 ). Therefore, the exclusion criteria is a mechanism to reduce potential study risks and reduce potential confounding variables ( Panacek & Thompson, 2007 ). Confounding variables may affect the outcome of interest (dependent variable), as in the sample study, the triglyceride level is the dependent variable. Therefore, potential participants using fish oil are not eligible to join.

In formulating the exclusion criteria, Hulley and colleagues (2013) recommend considering factors that increase the likelihood of loss to follow-up, the inability to provide necessary data, and the high risk of possible adverse effects related to co-morbidities or current treatments.

In continuing the HIV and high triglyceride example, participants actively abusing alcohol and illicit substances are not eligible to join this study. The chronic use of some substances can lead to changes in the brain and linked with the development of mental health issues such as paranoia, hallucinations, and other problems (National Institute on Drug Abuse (NIDA) ( NIDA, 2017 )). The use of illicit substances can potentially limit a participant's ability to comprehend and comply with the study requirements. The use of a measurement tool that examines attention, recall, and calculation and excluding participants who score below a required screening threshold (less than 23 on the Mini-Mental State Examination ( Folstein et al., 1975 )) is one method to screen for cognitive impairment and hopefully increase the likelihood for obtaining the required data. Participants with diabetes mellitus are excluded from the study to prevent possible adverse effects since fish oil can raise blood glucose levels and not safe for such participants.

Practical Considerations for Designing the Eligibility Criteria

For a new clinical research study, reviewing published studies with similar study participants and objectives can assist in formulating the eligibility criteria. Examine the approaches used in such investigations. Assess the data on the retention of the participants and evaluate the reasons for lost-to-follow-up. Much can be learned from the experiences of previously conducted studies to assess if methods were successful and allow investigators to apply useful strategies to their new study.

Defining the eligibility criteria requires a delicate balance between restrictive and less restrictive. A restrictive criterion can make participant recruitment and enrollment more difficult since the study requirements can reduce the eligible participant pool. Additionally, the restrictive criteria might impact the time-frame to complete the study, as well as the workforce needed to recruit and to enroll study participants, requiring more time and staff to complete. On the other hand, a less stringent eligibility criterion is often easier to recruit and to enroll participants and can increase the generalizability of a study, but can leave it open to more confounding variables and a heterogeneous participant pool ( Gallin et al., 2018 ).

During the development of the eligibility criteria, review the Code of Federal Regulations (CFR) 45 , which focuses on the protection of human subjects. Since institutional review board (IRB) approval is mandatory for all clinical research studies, IRBs will assess study protocols to ensure it meets the CFR 45 requirements before approving. CFR 45 focuses on the unbiased selection of study participants centered around gender, race, ethnicity, religion, nationality, and other factors ( Gallin et al., 2018 ). CFR 45 stipulates that all eligible study participants are considered unless there is a compelling safety or scientific rationale for excluding them ( Gallin et al., 2018 ). For example, a prostate cancer study can reasonably exclude women since the disease under study is gender-specific. A case to prohibit pregnant women might involve studies with known risks to maternal and fetal health.

In writing the eligibility criteria, use bullets or a numbered list ( Gallin et al., 2018 ). This format makes it easier for IRB review and implementation. A printed or an electronic version of the eligibility criteria placed near the telephone, computer, or participant screening area can provide an excellent guide for study team members to review the study criteria and reduce the chances for incorrectly enrolling ineligible participants.

Study Recruitment

Formulating the recruitment process occurs after the development of the eligibility criteria. The study protocol provides the details of the procedures the research team undergoes to recruit and to enroll study participants. In designing the recruitment plan, consider the anticipated participant accrual rate, which includes the number of participants enrolled per month/year and how long it will take to complete the recruitment goals. Additionally, other critical components of the recruitment process include the identification and the recruitment of participants, and the methods to advertise the study and the evaluation of the recruitment plan ( Gallin et al., 2018 ; University of Rochester, 2018 ).

Describing the specifics of how and where to obtain eligible study participants are also essential components of the study recruitment section. Details include the methods to approach the participants and the setting (i.e., clinic setting, research site). For example, in recruiting for the HIV+ adults with high triglycerides, flyers, and brochures were developed and sent to local HIV clinics in the New York City (NYC) area to promote the availability of the study. Potential participants called the research center to inquire about the HIV study; hence the participants initiated the process to join the research study.

Recruitment materials should include the title of the protocol, IRB study number, the sponsoring institution, contact information, purpose of the research (i.e., HIV+ adults with high triglycerides) and indicate whether compensation for time and effort is given to the study participants ( University of Rochester, 2018 ; Gallin et al., 2018 ). All methods to advertise or promote the study requires IRB approval before implementation. Some examples of recruitment materials include the telephone script used to discuss the study, and flyers, posters, postcards, newspaper advertisements, press releases, website advertisements, electronic mailings and social media blogs, and tweets ( Gelinas et al., 2017 ; University of Rochester, 2018 ). Each IRB may require specific information for recruitment materials to obtain approval.

Most institutions publish their IRB requirements online; therefore, to ensure a smoother review process, read the guidelines associated with each IRB institution. Participant recruitment is often one of the most challenging aspects of conducting clinical research. Delays in participant enrollment account for up to 60% ( Gelinas et al., 2017 ) of issues encountered in clinical research; therefore, a well-thought-out and feasible recruitment plan is vital.

The development of the eligibility criteria is a complex process that can affect participant recruitment, enrollment, and study completion. Reviewing the literature for research studies similar to the proposed target population is an excellent method to identify appropriate eligibility criteria. Developing a feasible recruitment plan in terms of selecting the suitable participant pool and approaches to recruit and enroll the eligible participants is at the cornerstone for implementing a successful study.

Acknowledgments

This manuscript is supported in part by grant # UL1TR001866 from the National Center for Advancing Translational Sciences (NCATS), National Institutes of Health (NIH) Clinical and Translational Science Award (CTSA) program.

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