Jul 10, 2019

Imagine you are preparing for a human factors (HF) validation study to bring your medical device to market, and the market research recruiters inform you that it will be very challenging, if not unfeasible, to recruit the required 15 participants per user group. What do you do?

It is not uncommon for some user populations, especially patient populations with rare or debilitating conditions, to prove difficult to recruit. As such, it is important to understand the various considerations for successfully and effectively including representative patient populations in HF validation studies.

Patient characteristics

In some cases, it might be necessary to include “proxy” users—individuals who are not intended users of your product, but whose characteristics align with those of your intended users. In these cases, we know that it is important to compare multiple characteristics (e.g., age, education, literacy level, sensory or physical impairments) to ensure the proxy users are representative of intended users.

Investigators should also look beyond the physical and demographic characteristics and consider intended users’ mentality towards a specific chronic condition. For example, imagine you are developing an autoinjector used to treat Growth Hormone Deficiency (GHD) in adolescents, but you’re having difficulty recruiting adolescents with GHD because of its relatively low incidence rate. Typically, GHD does not result in sensory or physical limitations, so it appears on the surface that the main characteristics you should look for in the injection experienced proxy users is experience treating a condition using injection devices.

In this case, adolescents with Type 1 Diabetes (T1D) who have experience injecting insulin might be a good fit. However, a closer investigation might reveal that the mentality T1D patients have towards using an injection device varies greatly from that of GHD patients: T1D patients inject insulin multiple times each day, and their lives are dependent on them performing their injections correctly. On the other hand, GHD patients might only inject once every two weeks, and their lives are likely not at risk if they skip an injection or inadvertently administer an under dose. These differences in commitment to and experience managing the respective conditions would likely result in GHD and T1D users approaching tasks differently during the study. As such, T1D patients might not actually be representative users for injection-experienced GHD adolescents, resulting in the need to include actual GHD patients. On the contrary, for the likely other user group of injection-naïve GHD adolescents, any adolescent who does not have injection experience would likely qualify as a proxy user.

As you can see, there any many complexities associated with identifying proxy users to include in an HF validation test. As such, we always recommend submitting your HF validation protocol for FDA review before conducting the study to confirm that the agency agrees with your recruiting approach.

Patient conditions

Taking it a step further, HF validation studies should include use scenarios that are as real as possible. This includes ensuring that, to the extent possible, patient participants experience all feelings and symptoms that they would during the time of actual product use. For example, if your product is a medication used to treat Parkinson’s Disease (PD) patients when their primary medication is wearing off (i.e., when PD patients are in the “OFF” state), PD patients will likely be experiencing more pronounced PD-induced impairments during real use. As such, you might consider conducting the study sessions when PD patients are in an “OFF” state, requiring PD patients to withhold their medication for a few hours leading up to and during the study.

Noting that PD patients would need to deviate from their standard medication regimen for the study, keep in mind that it’s your responsibility to pay special attention to human subjects protection. In this case, consider hiring a specialized clinician to remain on-site, or conducting the study at a clinical site, in the event that the PD patients need any medical assistance while off of their primary medication.

Tips for recruiting patients

Thinking back to our GHD example, what if all else fails? It is possible that you might not be able to identify suitable proxy users, requiring you to get creative with your recruiting approach. Here are a few methods we have used in the past to identify difficult-to-recruit individuals:

  • Contact support groups. These days, many support groups exist, particularly for under-represented individuals with rare or debilitating medical conditions. A Google search might reveal local or regional chapters, as well as upcoming events that attract individuals from different geographic regions. As an example, we conducted focus groups involving people with Transthyretin-Related Amyloidosis (ATTR)—a rare genetic condition—to gather information related to user needs. For this research, we established a relationship with a support group and traveled to an American Indian reservation where the group was hosting their yearly advocacy meeting. Here, we found that patients were very willing—even excited—to participate in our research.
  • Engage clinicians. We often have success seeking the support of clinicians who treat the desired patient population. In these cases, the clinicians are typically willing to inform patients or post fliers around their offices advertising the upcoming study. This method proved to be effective recruiting individuals such as people with Juvenile Idiopathic Arthritis (JIA), adolescents with immunoglobulin (IgG) deficiencies, and adults with Parkinson’s Disease.

Ella Engels is Human Factors Specialist at Emergo by UL’s Human Factors Research & Design division.

Related human factors engineering and usability resources from Emergo by UL:

  • Human factors user research for medical devices and IVDs
  • Human factors analysis support for medical device companies
  • Webinar: Meeting US FDA expectations for human factors engineering activities
  • Whitepaper: Applying human factors to wearable medical devices


  • Ella Engels