Introduction to Human Factors for Medical Devices

April 15, 2021
Thoughts by
Tyler Hagler

Defining human factors is a mess. The field is inclusive of psychology, cognitive science, operations engineering, decision making, workplace layout, and various engineering domains like aerospace and transportation - these fields extract and contribute to the domain of human factors.

The Domain of Human Factors

The list of fields represented are so diverse that it would be helpful to provide a more concise definition:

Human factors is how we apply knowledge of humans to the design of things.

Knowledge of humans, in terms of psychology, anthropometry, biomechanics, and ergonomics is one large domain.  The design of things is another large domain that includes industrial design, engineering, and marketing.

There are a lot of topics that can be included with human factors beyond medical devices and biomedical engineering. When we constrain the field down to what the FDA cares about, the guidance documents are primarily concerned with usability. Is the device demonstrated to be safe and effective for the intended users, uses, and use environments? The FDA isn’t as concerned about mandating ease of use for the device, except where it might impact safe and effective operation on critical tasks. The goal is to ensure that the device user interface has been designed such that use errors that occur during use of the device that could cause harm or degrade medical treatment are either eliminated or reduced to the extent possible. Human factors for medical devices are substantially bracketed down from an otherwise broad domain of knowledge.

Usability is a method within human factors to understand how humans interact with a specific product to understand how to make it better.

With usability studies, we set up simulations with the intended users, use cases, and use environments to test prototypes. The study might simply establish a baseline for how difficult or unintuitive the device is to operate and identify critical tasks that can pose a risk or hazard to the patient.

Ergonomics is the physical study of how the variety of human bodies interact with physical products.

When you think about the variety of sizes of human bodies, it spans 5th percentile females up to 95th percentile males. This range affects the span of hand sizes for controls and device grips as well as standing or sitting height adjustments. It is a common mistake to focus on the middle of the bell curve and only study the 50th percentile average human. It is on the edges of the bell curve where true inclusion can be found for humans of all shapes and sizes.

Task analysis is a user-centered approach to the design of the device.

What are all the tasks needed to be performed by the user? This is an analysis of all tasks from unboxing to disposal of the device. The task analysis often brings up new hazards that haven’t previously been identified in the risk assessment. A detailed task analysis for complicated devices provides an excellent metric for improvement as design changes chip away at simplifying a 200-step process.

Mental Model is the cognitive model that the user is primed to apply to the use case and use environment.

If a user walks into a dark room, their mental model for turning on the lights is to flip a switch up at a wall plate positioned between shoulder and waist height near the entrance of the room. We would break the user’s mental model, for example, if the light controls are tied to a foot pedal positioned under a table. The design of things can get more complicated when different users bring different mental models to the device. A user who is an avid golfer will approach a device with very different mental models for hand controls than a user who frequently uses game console controllers.

PCA elements: Simple tasks like turning the device on might appear simple, but there might be sub-tasks that fall into Perception, Cognition, and Action elements of any task. Have to see the button, understand that it is a button, and push the button.

Critical Tasks are tasks that, when performed incorrectly, pose a risk or hazard to the patient and users. These tasks are documented both in the task analysis and risks and hazards analysis. The design history file needs to show a reduction in risk for critical tasks for FDA approval.

Formative is an evaluation of a prototype with a small sample, but more than one user. The goal is to learn and make the product better.

It is important that the sample size is more than one user, preferably someone who hasn’t been part of the design team yet is trained in the use case. The variety of users can help identify mental models, ergonomic fit, and other factors that are only emergent in response to a prototype by the intended user.

Validation / Summative User Test is the culmination of all the formative tests to validate that the device can be used by the users in the intended environment as intended. If the design is what it should be, then the usability is proven out without prompts from the researchers. Researchers should do their best to sit back and shut up during a summative user test to avoid influencing the respondent beyond what they might encounter in the intended use environment.

Is human factors always integrated with the process of medical device design?

The field of medical device design has changed dramatically over the past 10 years as the FDA human factors guidance documents have come into play. Any device that has a use case with critical tasks that, if the user fails to perform the task, could lead to risk and hazards for the patient, then the FDA will not approve it.

When should designers start to incorporate human factors into the design process?

If you’ve got humans involved with your device, then human factors should be considered as early as possible in the design process. Where a device design can get into trouble is if the technology decisions are being made at an early stage that creates an avoidable human factors problem. User research, industrial design, and technology development should progress simultaneously at similar levels of definition over the course of a medical device design project. If any one of those elements gets ahead of the other, then the entire program can get out of balance. It’s not just adding in human factors to a technical solution. Sometimes a technology solution enables smaller, lighter, better performing design elements for the whole system that provides an opening for designs that weren’t possible before.

What happens if I present my design to users and they don’t use it as intended?

In this situation, it isn’t immediately clear if you have use error or user error. Too often, we assume user error, where the user didn’t read and remember every nuance in the instructions for use document. With use error, it could be that you are asking the user to turn on the lights by hitting a foot pedal under the table when their mental model is to turn up a rocker switch by the entry door to the room. The design of things to be intuitive and easy to use involves creating interfaces that invite proper use. Design teams should only rely on the instructions for use as a last resort after having exhausted exploration of inviting and intuitive user interfaces. Instructions for use is a lazy solution for correcting user error.

Tyler Hagler

As a career industrial designer and innovation practitioner, Ty Hagler has managed hundreds of new product development programs through the process of opportunity identification guided to commercialization.


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