This systematic review of the literature confirms that while numerous recruitment interventions for this population have been assessed, methodologic rigor is variable and the body of evidence has significant gaps. The heterogeneous nature of the trials’ interventions, target populations, and study designs rendered data synthesis challenging. Data on relative effectiveness of different recruitment approaches would be optimized if future studies could be designed to enhance internal and external validity as well as the transparency of reporting.
Some of the more commonly attempted recruitment interventions for minority populations included church recruitment and interaction with community organizations and leaders. Health system recruitment, including registry/chart review and physician referrals, was also assessed in about half of the studies. Given the recent emphasis on community-based research, it was surprising that health system recruitment was such a commonly used recruitment approach.6,7,9,10
This may have arisen from the perceived importance of physician referrals as gatekeepers to minority research recruitment, which has been previously documented.4,11,15
There was no clear dominant strategy. In fact, social marketing, health system, and referral interventions led to the highest proportion of participants 44, 40, and 35% of the times in which they were tried, respectively. The main outlier was community outreach which was the most successful in a mere 13% of the studies in which it was assessed. Although heterogeneity of studies and populations renders definitive comparisons difficult, the poor showing of community outreach merits further exploration as this is a commonly proposed intervention.3,7,9,10,22,72
It is important to note that community outreach likely has the advantage of being the recruitment strategy that may alleviate distrust.3,6
Our finding that outreach rarely was the most effective independent strategy may stem from the individual studies/populations in our review. Alternatively, perhaps the benefits of a collaboration with the community cannot be quantified as an independent strategy, but rather are most effective when incorporated into a comprehensive approach that includes other recruitment interventions.
We were surprised to note that some commonly proposed recruitment incentives such as financial reimbursement (cash or coupons) or convenience strategies (transportation/flexible scheduling) were infrequently assessed. Although we identified 9 studies that used financial incentives to assist in recruitment, none of them compared the effectiveness of this incentive with other recruitment interventions. For instance, Vollmer, et al. indicated that only 6 and 20% of African Americans and other minorities, respectively, cited financial incentives as their primary reason for participation, although recruitment fraction or enrollment proportions could not be calculated.62
This is an important gap in the literature. Financial incentives are commonly recommended, but they are not without controversy. Some authors have raised concerns that payments to underserved populations represents coercion, while others have suggested that payments may bias study results obtained or population recruited.73–79
Accordingly, payments could potentially have a negative impact on trust, especially in vulnerable populations that might already be distrustful of research.7,77
Our review also demonstrated that both the reporting and the methodological rigor of recruitment intervention studies has room for improvement. Inclusion of quantitative screening and recruitment data in manuscripts would allow for a more transparent assessment of which recruitment intervention lead to the most subjects screened (greatest reach), most eligible participants (relevant reach), and enrolled. A limitation for assessing media and community outreach recruitment includes the difficulty in ascertaining the number of people who were exposed. While it would not be feasible to determine the number of people who had seen or heard a specific advertisement, future studies should try to report the size of a target audience and frequency of exposures at the population level as well as determining the efficacy of interventions across racial and ethnic groups. Although this was beyond the scope of our study, the comparison of recruitment interventions across different target populations (such as different racial or ethnic groups) should be performed when data are available because the effectiveness of interventions may be moderated by cultural and contextual effects.
Additionally, it is important to report the amount of time and cost per recruitment intervention (which was done in fewer than 15% of studies in our sample) as this could affect decision making for researchers and funders. Finally, authors need to report if the different populations they are recruiting via different recruitment interventions are balanced with regard to sociodemographic or clinical characteristics and if they are representative of the larger population from which they were recruited.
In addition to the quality of reporting study data, we found room for improvement in the internal validity of many study designs. A control recruitment group was lacking in about one-third of our studies, and those that did employ a control group rarely used randomization as a means to diminish bias. Furthermore, with only 21% of the studies using formal statistical analysis, it was unsurprising that only a minority of studies had the data to support the authors’ conclusions. When the validity of authors’ conclusions and study outcomes are questionable, interpreting the data presented by such studies is difficult.
An important caveat when considering the frequency of “statistically significant” findings across studies is the variation in sample size. However, we found no systematic relation between sample size, study design, and outcome in our dataset (data not shown). Finally, the 4 categories of recruitment approaches included in this study were intentionally rather broad so as to be inclusive and concordant with the depths of reporting provided by the manuscripts. We recommend that future studies report data per specific recruitment intervention to allow for more detailed analyses of the success of various approaches.
Approaches that are effective in 1 setting or 1 population may not be generalizable. We were therefore concerned that there was a paucity of studies that focused on Asian Americans, Native Americans, and other minority populations, as researchers have little evidence to guide their efforts for these populations. Another population that merits further research is the minority elders. When trying to enroll these individuals, researchers face the challenges associated with the inclusion of minority participants as well as those encountered when recruiting older adults into disease-oriented clinical trials such as a higher prevalence of comorbid illness and functional impairments.80,81
Additionally, many of our parent studies were prevention rather than treatment trials. Hence, our findings may not be applicable to other populations, types of studies, or disease entities, as each of these factors may have different barriers to recruitment.25
Finally, our sample was restricted to manuscripts that reported recruitment data; these manuscripts represent a minority of the clinical trials conducted and published. Many other studies may have used or evaluated different recruitment approaches but did not report results.
It is important to identify not only successful recruitment interventions but also efficient ones. We did not find any evidence that social marketing was any less efficient—in terms of the proportion of screened patients who eventually enroll—than any other intervention. Although social marketing appears to yield roughly the same proportion of study subjects as health system and referral approaches without lower recruitment fractions, further studies are needed to determine whether its efficiency as a recruitment intervention is comparable to other strategies. Researchers need to weigh their resources of time, financial budget, effort, and expertise to choose which of the recruitment interventions are truly ideal for the population they are attempting to recruit.
At a time when minority recruitment for research continues to be an important issue that many still find challenging, it is vital to develop and evaluate innovative approaches to increase minority participation. For instance, the National Cancer Institute’s Minority-Based Community Clinical Oncology Program (MBCCOP) has enrolled thousands of minority patients into cancer clinical trials.82
This suggests that minorities can be recruited when appropriate outreach and infrastructure mechanisms are supported. While we found that there is an increasing body of evidence regarding approaches to enhance equitable recruitment, more work is needed. Improved methodologic rigor and data reporting are paramount to continue assessing successes or failures of recruitment interventions in such special populations. Perhaps, with rigorous evaluation of recruitment methods and adequate funding for strategies that are found to be effective, the goals of the NIH with respect to equitable and representative access to research studies can finally be realized.