Usability engineering is about understanding who uses the device, the tasks of the users, the environment in which they will use it, and designing a great user interface.
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A user interface is so much more than just a screen and buttons. When talking about a medical device, all kinds of accompanying documentation and virtually everything that the user can interact with needs to be considered a user interface. It even includes auditory signals and the instructions for use.
Preparing a use specification
Usability engineering should start with the preparation of a use specification. This is a document that includes all the information about the users of the device in question. It can be seen as an extended form of the intended use – getting that information right is crucial for the success of the product.
Different users and use environments
Let us take a glucose measuring device as an example. If it is used in a home use environment by a young child, a different device will be needed compared to a device that will be used by a professional caregiver in a hospital environment.
The same could be true with a home-only use environment. A device that is used by children only will probably be designed differently than a device used by elderly people only.
To make sure that the device will be good and safe, identifying the right user profiles and writing down all the information that makes the user so special is of great importance. This includes not only characteristics of the person, but also characteristics of the task and the use environment.
Analyse the safety of the device and user interface
Based on what the use and the users are like, what should be analysed is what could go wrong with regards to the safety and user interface. This is similar to identifying hazards or answering questions relating to the safety of the device that many would be doing as part of a risk management process according to the ISO 14971 standard.
Safety plays a major role in usability engineering for medical devices. One term that inevitably comes up is use error.
Use error has a very broad definition. One could say that a use error happens every time a human and a machine “misunderstand” each other. Not every use error will lead to a hazardous situation, but some use errors may, and those are the ones usability engineering focuses on.
A use error is defined as:
“a user action – or a lack of such an action – while using a medical device, that leads to different results as intended by the manufacturer or expected by the user of the device”.
Typical examples of a use error could be a nurse who accidentally pulls out the cable of the sensor of a monitoring device without noticing it, or a healthcare professional who touches a sterile tissue with their bare fingers, before applying it to a wound. It could also mean that someone accidentally connects a cable to the wrong port of a medical device and the signal is not transduced correctly.
It is crucial, though, to differentiate between a use error and a user error. User error implies that it is the user’s fault, but it is most definitely not always the user’s fault in case of a use error and the term user error should not be used.
Hazard-related use scenarios
Another important term that is highly relevant when talking about things that could go wrong is the term hazard-related use scenarios. This term can be challenging for those that are not always working with risk management or usability engineering.
A hazard-related use scenario is a hazardous situation that occurs as a result of a use error. All hazard-related use scenarios need to be identified and documented. Based on this, it is decided what to evaluate at the end of product development. They will be included in the summative evaluation to ensure that the medical device is safe.
Proving or verifying that the device is safe is a must because it is a general principle in both risk management and design control. The success of risk control attempts is checked by the summative evaluation.
User interface requirements in usability engineering
When implemented, user interface requirements should reduce the risks that were previously identified to make sure that when the final evaluation of the device is done, the user will not end up in situations where the risk is unacceptable.
From a risk management point of view, one could say that these requirements are risk control measures, and that is the great thing about starting with the risk in mind: it is known which user interface requirements need to be included in order to reduce those risks.
Nevertheless, usability engineering should not only include requirements that make a safe device, but also the requirements that come to mind when thinking about the use environment and the tasks of the users.
Formative evaluation in usability engineering
The next step is implementing the requirements by designing the device and its user interface. Formative evaluations should be done along the way, and those are tests that help the manufacturer check that they are on the right track. This is done because it rarely happens that the design is right on the first try, especially when designing a new product.
Formative evaluation methods
A formative evaluation is basically any method one can come up with, that would solicit feedback on the user interface. In its simplest form, it is asking if someone understands it, or what the symbols should be like on the device.
More advanced formative evaluations include testing in an environment that is as real as possible with multiple users and a product that is similar to the final one. The amount of work in this field depends on whether the product is brand new or not, and the severity of the harm that could occur as a result of a failure in the user interface.
If a new iteration is being made on an already existing product, there is likely going to be very little formative evaluation, or even very little usability engineering as well. On the other hand, when developing a brand-new high-risk product that needs humans to operate it correctly, a lot more time and resources will need to be invested in this area.
If the usability engineering team realise they are not on the right track, they will need to iterate on the design and improve. That is the way the development goes until the last formative evaluation shows that there is nothing left to improve.
Summative evaluation in usability engineering
After the formative evaluation and the completion of the design, the summative evaluation takes place. The summative evaluation should demonstrate that the user does not end up in a situation with an unacceptable risk as a result of poor usability.
It is important to note that summative evaluation is not the same thing as design validation. The design validation will focus on showing that intended use and user needs have been met, while the summative evaluation is about showing that the user interface is safe.
In conclusion, summative evaluation is testing or verifying that the usability under conditions of simulated use. It should also be done using production units or equivalent devices. From that point of view, it is similar to design validation, but the purpose is different. All the documents and records created throughout the usability engineering process are part of the usability engineering file.
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Dr. Michaela Kauer-Franz is a highly esteemed trainer, consultant, and entrepreneur in the medical device industry. She has vast hands-on experience, having developed, amongst other things, monitoring devices for ICU and NICUs, diabetes care products, and women’s health products.
Michaela Kauer-Franz has received numerous awards including the Red Dot Winner Interface Design Award and being nominated for the UX Design Award. She is a lecturer for the design of medical and critical user interfaces at the Technische Universität Darmstadt and part of the national standardization committees in Germany for Usability and User Experience.