I. What is usability engineering in medical devices?
Usability engineering refers to a process that helps identify acceptable safety risks related to the usability of a medical device. However, it’s important to know that the scope of usability engineering is larger than safety. That’s because usability engineering is also concerned with how well and easy a user can interact with a specific medical device to achieve the desired outcomes. Thus, usability engineering also affects customer experience and satisfaction.
Manufacturers of medical devices should start the usability engineering process from the start of the product development phase. It should be noted that to obtain FDA approval, manufacturers must also provide a summary of how usability engineering was executed throughout the product design phase.
II. What are the standards for medical device usability engineering?
The leading safety standard for medical device usability engineering is IEC 62366. It details a process that helps manufacturers assess and mitigate risks associated with the correct use and use failures. The standard excludes abnormal use and malice – check the medical device risk management vocabulary to learn the meaning of these words or any other.
This standard consists of two parts. Part I, IEC 62366-1:2015, specifies a process that a device manufacturer can follow to develop, examine and evaluate a medical device’s usability. Manufacturers who wish to obtain CE marking should comply with the harmonized version of this standard – EN 62366-1. Part II of this standard – IEC TR 62366-2:2016 – represents a technical report. It contains background information and provides guidance on implementing the usability engineering process.
III. Importance of task analysis for usability engineering
The safety standard for usability engineering, IEC 62366, requires manufacturers to identify user interface characteristics that could affect the safety of medical products. A common tool to accomplish this is task analysis. The latter represents a formal and systematic activity that starts by creating a detailed description of simultaneous actions of the medical device user.
Task analysis typically begins with high-level use scenarios, later adds tasks, and eventually details the individual steps. It must include information on the user-performed steps and how errors in performing these steps could result in hazards. The task analysis should also include conditions of use and user profiles. Lastly, its results should be presented in a table or flowchart.
IV. Importance of the user-device interaction model
The user-device interaction model helps manufacturers predict potential use failures that could cause a hazardous situation and identify the necessary human capabilities for interacting with the medical device. The model of user-device interaction includes two parts:
- The user enters an incorrect input into the medical device via the user interface, which in turn creates a hazardous output.
- Through either a perception error (e.g. seeing and feeling) or a cognition error (e.g. interpretation), the user takes an action (e.g. touch and press) that creates a hazard.
Use failures are usually the result of a contradiction between the mental model that a user has and the actual behaviour of the systems. They can be caused by one or several of the following design factors:
- Ambitious or unclear device settings
- Insufficient visibility, tactility or audibility
- Poor mapping of controls to actions
- Poor display of information related to the actual state of the device
- Complex or confusing control systems.
Use failures can also be due to environmental factors such as temperature, acoustic noise, physical surroundings, humidity, etc. However, it should be noted that not all use failures result in a hazard. The BXM method can help with the identification of product hazards that are the result of use failures.
V. How to control usability risks?
Generally, manufacturers can perform design changes to reduce the usability risks due to use failures. For instance, the following design means can be considered:
- Keystroke debouncing: If the same key is pressed within 200 ms, ignore the following keystroke.
- Proper sizing: use anthropomorphic data to size the user interface so that physical errors are less likely.
- Font size for visual displays: refer to standard AAMI HE75 for guidance.
- Reasonableness checks: evaluate the user input for reasonableness.
- Include alarm types: refer to standard IEC 60601-1-8 for guidance.
If any of the abovementioned design controls are employed, the same should be verified for effectiveness in risk reduction.
VI. Usability engineering data gathering
Manufacturers of medical devices based on existing released products, for which there is postproduction data available, can use the available data to extract P1 data (i.e., the probability of occurrence of hazardous situations due to use failures). Afterwards, they can use P1 in conjunction with P2 data (i.e., the likelihood of experiencing harm from a hazardous situation) to estimate the usability risks due to use failures.
However, if the medical device is new and unique, or part of its user interface is new, or postproduction data is unavailable, the above doesn’t apply. In such cases, the manufacturer needs to plan and execute formative and summative studies to generate the necessary data to support P1 estimates. Then, they need to compute the risks of use failures using the P1 data.