Usability Engineering

Every usability programme starts by clearly defining:

• Intended users: Identifying the intended users of a medical device is one of the most important steps in usability engineering. This involves defining the specific groups of people interacting with the device during its lifecycle. Intended users can include trained healthcare professionals, such as surgeons, nurses, paramedics, lab technicians, and lay users like patients, caregivers, or family members. Each group may have different levels of knowledge, skills, experience, and physical or cognitive abilities. For example, a surgeon operating in a sterile field has very different needs and expectations from a diabetic patient managing their insulin therapy at home. Recognising these differences helps ensure that the device design and instructions are tailored appropriately to minimise use errors and support safe, effective operation.
• Intended purpose: A device’s intended use (or intended use) describes its medical purpose and how it is meant to be used. This includes the specific indication (e.g. monitoring blood glucose levels, delivering insulin, suturing tissue), the conditions under which it should be used, and any limitations. Defining the intended purpose helps determine the critical tasks users must perform and informs risk management. It also helps identify where use errors could occur, especially if users attempt to use the device outside its intended purpose (known as reasonably foreseeable misuse).
• Use environment: The use environment refers to the physical, environmental, and organisational settings where the device will be used. This can range from highly controlled environments like an operating room or hospital ward to more variable and unpredictable settings such as a patient’s home, a moving ambulance, or a field clinic. Each environment brings different challenges. For example, home users may face poor lighting, clutter, or distractions; emergency responders may need to operate the device in noisy, high-pressure situations; hospital staff may need to use it while wearing gloves or other protective equipment. Understanding the use environment helps manufacturers design devices and instructions that account for these real-world conditions, reducing the risk of use errors and ensuring the device functions as intended.

In parallel with risk management activities conforming to [ISO 14971], manufacturers must systematically identify and evaluate how users interact with a medical device to ensure that the design minimises potential harm. Usability engineering and risk management are closely intertwined because many device hazards arise not only from technical failures, but from use errors—that is, mistakes or incorrect actions taken by users that could lead to patient injury or compromised treatment.

Two key concepts in this process are critical tasks and potential use errors.

• Critical tasks: A critical task is any user interaction with the device where a failure to perform the task correctly, or an omission of the task, could result in serious harm to the patient, the user, or others. Identifying critical tasks allows manufacturers to focus design and validation efforts on the parts of device use that present the highest risk if not performed correctly. Regulators expect that these tasks be carefully analysed, and that final usability validation demonstrates that typical users can complete them safely and effectively.
• Potential use errors: A potential use error refers to any action or lack of action during device use that could result in unintended outcomes. Use errors can arise from a wide range of causes, such as poor interface design, confusing instructions, environmental factors (e.g. lighting, noise), or limitations in user knowledge or abilities. Identifying potential use errors helps manufacturers implement design solutions that prevent errors or mitigate their consequences. Techniques like task analysis, use error analysis, and use-related risk analysis (URRA) are commonly used in this step.

Designing for usability is about creating medical devices that people can operate safely, effectively, and with minimal effort or risk of error. This requires more than good intentions—it calls for structured approaches that put users at the centre of the design process. These activities can be integrated into Design controls within overall product development.

Applying human factors principles

Human factors principles are guidelines and best practices derived from understanding how people perceive, think, act, and make decisions when interacting with devices. In medical device design, this means:

• Ensuring clear, unambiguous labelling that supports correct use and highlights critical warnings or instructions. Using standardised symbols (e.g. ISO 15223-1) reduces reliance on text and minimises translation requirements.
• Designing intuitive controls that align with user expectations (e.g., buttons that are sized appropriately, placed logically, and behave consistently). New features must be intuitive without introducing confusion. Designers must balance innovation with users’ mental models and existing practices.
• Considering ergonomics, such as the force needed to press a button or the visibility of a display in different lighting conditions.

By applying these principles, manufacturers reduce cognitive load, prevent confusion, and lower the likelihood of use errors.

Prototyping and gathering user feedback early

Prototyping is a powerful tool for embedding usability into device design. Early prototypes don’t have to be fully functional; even basic mock-ups or 3D-printed models can reveal much about how users perceive and interact with a product. Prototypes allow teams to:

• Test ideas before committing to costly development work.
• Identify unexpected challenges or user preferences.
• Explore alternative solutions quickly.

By gathering feedback early in development, manufacturers can refine features, layouts, and interfaces to truly meet user needs. This iterative approach is more effective than trying to “fix” usability issues late in development.

Using simulations or mock-ups to test design ideas

Simulated use studies or mock-ups provide a safe, controlled way to observe how real users interact with a device in realistic scenarios. These simulations might:

• Recreate typical use environments, such as hospital wards, home care settings, or ambulances.
• Include time pressures, distractions, or stressful conditions that mirror real-world use.
• Allow users to perform critical tasks while designers observe and collect data on success rates, use errors, or hesitation points.

Such evaluations help uncover usability issues that might not appear in theory or on paper. For example, a control that looks fine on a CAD drawing might be hard to reach or easy to confuse with another button when wearing gloves. By testing in simulated conditions, manufacturers can catch and address these problems before the device reaches the market.

Formative usability studies are an essential part of the usability engineering process for medical devices. Their main purpose is to explore and refine design concepts early and throughout development, helping manufacturers identify and address potential use-related risks before committing to a final design.

These studies are usually conducted iteratively, allowing teams to learn from user interactions, make informed design decisions, and progressively improve the device’s usability. Because formative studies are part of the development phase rather than final validation, they generally carry a lower regulatory burden and are focused on gaining insights rather than demonstrating compliance.

Common types of formative studies include:

• Interviews: User interviews can provide valuable qualitative data about intended users’ needs, experiences, preferences, and challenges. These interviews might explore how users currently perform relevant tasks, their mental models, or their expectations of how the device should function. Interviews can also uncover contextual factors that might affect usability, such as environmental conditions, physical limitations, or workflow constraints. Early-stage interviews are especially useful for informing initial design requirements and identifying critical tasks.
• Heuristic reviews: In a heuristic review (or expert review), human factors specialists or usability professionals evaluate the device design against established usability principles or heuristics. This method doesn’t involve end users directly but provides an expert perspective on potential usability issues. For example, reviewers might assess whether controls and displays are consistent, whether the design minimises memory load, or whether error messages are clear and actionable. Heuristic reviews are a cost-effective way to identify obvious usability flaws before involving end users in testing.
• Simulated use scenarios: In simulated use scenarios, representative users attempt to complete tasks using device prototypes or mock-ups in settings that approximate the intended use environment. These scenarios can help identify where users struggle, misunderstand instructions, or make errors. Because the scenarios are not part of formal validation, the prototypes don’t need to be fully functional, and scenarios can focus on specific interactions or tasks. This approach is especially valuable for observing real-world behaviours and refining design elements like labelling, control layout, and workflow.
• Usability walkthroughs: A usability walkthrough is a structured activity where users or experts step through tasks with a prototype or design concept, describing their thought process and expectations at each step. The goal is to uncover areas where users might become confused, make mistakes, or feel uncertain. Walkthroughs can reveal mismatches between users’ mental models and the device design, enabling teams to address those issues early. They also provide insight into how well the device supports intuitive operation.

Formative usability studies are intended to be exploratory, flexible, and iterative. They allow manufacturers to test ideas, learn what works and what doesn’t, and adjust their designs cost-effectively with low risk. By proactively addressing usability challenges during development, manufacturers increase the likelihood that their final design will pass human factors validation testing and, most importantly, be safe and effective for its intended users.

The final step in usability engineering is human factors validation testing, often called summative evaluation. This critical phase provides evidence that the medical device’s user interface is safe and effective for its intended users, uses, and environments. It is the culmination of earlier formative evaluations and design refinements, and it is essential for regulatory approval in most major markets.

Testing the final design in simulated conditions that reflect real-world use

In summative testing, manufacturers must evaluate the final, production-equivalent design of the device, including its labelling, packaging, instructions for use (IFU), and controls. The testing is done under simulated use conditions that closely mimic the actual environments where the device will be used. This might mean replicating the lighting, noise, time pressure, distractions, or emergencies that could realistically occur in hospitals, clinics, ambulances, or home settings. The goal is to capture how the user interface performs when subject to the same constraints and challenges that will exist in real life. Simulated use must strike a balance between practical constraints and realism. Overly artificial testing can miss critical usability issues.

Using representative users

It’s vital that summative testing involves real-world users who match the characteristics of the device’s intended users in terms of background, training, and experience. This means excluding designers, engineers, or company staff who are familiar with the device and would not use it the same way an untrained user would. The participants should reflect the diversity of the target user group, including potential variations in age, education, vision, dexterity, language skills, or experience with similar devices. If multiple user groups are identified (e.g. surgeons, nurses, caregivers, patients), each group should be evaluated separately. Finding users who accurately reflect the target population can be difficult, especially when devices are for specialised medical procedures or rare conditions.

Demonstrating that critical tasks can be performed safely and effectively

A central focus of human factors validation is ensuring that critical tasks—those tasks that, if performed incorrectly or not performed at all, could cause serious harm—can be carried out reliably. The testing must demonstrate that users:

• Can recognise and respond appropriately to safety information (e.g. warnings on labels or screens)
• Can correctly set up, operate, and shut down the device as required
• Can avoid foreseeable use errors that would lead to unacceptable risk

Manufacturers should define acceptance criteria for each critical task in advance and carefully record both successes and failures during testing.

Documenting results in the human factors engineering report

The findings from summative testing are compiled in the human factors engineering report (sometimes called a usability engineering file or human factors validation report). This report typically includes:

• A summary of the usability engineering process
• Identification of the device, user groups, intended uses, and environments
• Description of the summative test plan and methods
• Detailed results, including task success rates, observed use errors, and root cause analysis
• Justification of why any residual use-related risks are acceptable
• Conclusions regarding the adequacy of the user interface

This documentation is a key part of regulatory submissions, providing evidence that the device has been designed to minimise use-related risks. It should reference and relate to other technical documentation sections, especially the risk management file.

Regulatory expectations: demonstrating that residual use-related risks are acceptable

Regulatory authorities require manufacturers to show that any residual use-related risks that remain after applying risk control measures are acceptable and as low as reasonably practicable (ALARP) (although the definition of acceptable residual risk varies with device type and jurisdiction). This means:

• Risks must be reduced as far as possible through design, not just training or labelling.
• Any remaining risks must be clearly communicated and justified.
• The manufacturer must provide evidence that further risk reduction would not be proportionate or feasible without compromising the device’s benefit.

In some cases, regulators may request additional human factors validation data, especially for high-risk devices, combination products, or devices that involve complex user interactions.