Using the industry-known Jakob Nielsen’s 10 heuristics principles, I evaluated a feature in Fitbit Sense, a smartwatch released in September 2020. The goal was to systematically document usability issues that are present in wearable interfaces like this one and gather insights to improve future designs.

Following the approach recommended by the Interaction Design Foundation to conduct heuristics evaluations, I initiated the process by defining the scope of the assessment. Considering the broad range of features offered by Fitbit Sense (see Fig. 1), I focused the analysis on the stress management feature, which provides users with information on their stress level & recommendations on how to manage it by tracking electrical variations on the skin, called electrodermal activity (EDA) responses (see Video 1).

The next step was to determine the target users, their goals, context of use, and mental models. In this evaluation, I am both the user and the evaluator. To have a point of reference, I would match with users with this persona (see Fig. 2): female, aged in the range 30-35, married, young professional, with a post-graduate education, and living in Europe, in an urban zone. I am very interested in how technology helps improve people's quality of life, while at the same time respecting people's data and embracing sustainability principles. My goal using this smartwatch is fundamentally to learn more about my health and get empowered to do things to improve it. Following Lundell and Bates' classification of user journeys using smartwatches, this persona falls into the category "Tool Techie".

Finding usability experts who would also own the same type of smartwatch was difficult. For that reason, I am the single evaluator in this study. I am a Digital Product Designer with 9+ years of experience working as a Researcher & recently as a freelance UX Designer. I am trained & experienced in user research, usability testing, and the management of the smartwatch in the study.

After determining the usability criteria to evaluate (see above), I selected a ranking to assess the severity of the usability issues. Using a scale from 0 to 4 (see Fig. 3), ranging from no usability problem to serious usability issue, I assessed the feature in the smartwatch & complimentary mobile app. Then, I documented the observations and derived conclusions from the patterns identified.

This exercise helps to better understand what usability aspects are particularly relevant to wearable devices like smartwatches and allows to improve the user experience in future designs.

As indicated in the documentation and made visible in Fig. 5, usability heuristics #1, #8, and #9 require attention since they significantly affect the quality of users' interaction with the smartwatch & the app. The recommendations are:

• During the scan process, provide users with regular & subtle haptics (e.g., a pulse), to encourage them to be patient. An alternative could be to play background music, which could also aid users to reduce their levels of stress.

• Further summarize the background information provided. And lead users to the mobile app whenever more detailed information is presented to them. Also, bullet points, graphs, and icons can help increase usability, conveying more information in a shorter time and generating less cognitive load.

• Users should also be given an immediate possibility to re-start the scan or help fix it with, e.g., a longer scan process to gather the necessary data. In this way, we avoid making users wait until the end of the scan to realize that no data was captured to make a stress assessment.