AbstractMany interactive systems can be accessed across a range of different platforms, enabling cross-platform services and allowing users to migrate their tasks from one platform to another. Despite the increased worldwide use of cross-platform services, there is limited research into (1) how cross-platform usability can be assessed, (2) key, culture-related, and context-related user experiences (UXs) of multiple interactive systems across different platforms, (3) user interaction behavioural patterns when switching between devices and (4) the use of eye tracking for cross-platform usability evaluation. In this thesis, we present our findings from a set of cross-platform user interaction studies designed to answer research questions addressing these four areas. We used think-aloud protocol, observation, questionnaires and eye tracking to gather data. We defined cross-platform usability and developed a model for assessing it. In the first user study, we investigated our new model for assessing cross-platform usability, the key cross-platform UX elements and the user interaction behavioural patterns over three cross-platform services. Our analysis showed that our assessment model could be valuable for assessing and quantifying cross-platform usability. We also found that the data-collection techniques employed in our model including thinking aloud, observation and questionnaires allowed for the identification of a set of cross-platform usability issues. The think-aloud protocol generated the most cross-platform usability issues across all tested cross-platform services and tended to be the most valuable technique. Our analysis also showed that users were sensitive to the following six key cross-platform UX elements: consistency of system components across platforms, fluency in resuming interrupted tasks after transferring from one device to another, configuration of devices and functionality, service learnability, the extent to which user interfaces (UIs) supported the recognition of elements rather than recalling them after the transitioning process and the transparency of each UI. We also identified a set of cross-platform user interaction behavioural patterns, such as visual memory and habituation. The key UX elements and user interaction behaviours need to be taken into account when designing a better cross-platform UX. In the second user study, we investigated cross-platform cross-cultural UX. Forty students volunteered to participate in this study. Participants were from nine different countries. They carried out a set of tasks on five cross-platform services. Findings showed that users from the different cultures were sensitive to six cross-platform UX elements: consistency, fluency, configuration, learnability, recognition and transparency. This finding confirms the results of cross-platform UX elements in our first user study. Our analysis also identified a set of objective and subjective cultural factors that most influence cross-platform UX. The objective factors are service related (direction, translation, meaning of icons, format) and device related (typing interface design). These factors impacted how users perceived cross-platform services in terms of consistency and fluency. The subjective factors are power–distance (PD) and uncertainty–avoidance (UA). These factors impacted how users from different countries perceived cross-platform UX issues differently. The objective and subjective cultural factors need to be considered when designing international cross-platform services. In the third user study, we investigated cross-platform cross-context UX. Forty-five participants performed tasks on five cross-platform services across different contexts (in terms of location and situation). Participants were divided into ten different groups. Five participant groups switched between interfaces across devices in a seated–moving contextual setting. In this setting, participants used a laptop while seated in a lab environment and a mobile phone in a moving situation outside the lab. The other five participant groups attempted their tasks using cross-platform services in a seated setting. In this setting, participants used both a laptop and a mobile phone while seated in the lab. Our study findings showed that in each test setting, users were sensitive to five cross-platform UX elements: consistency, fluency, configuration, learnability and recognition. These five elements were also identified by the first and second user studies. However, our analysis showed that testing cross-platform UX in the seated–moving settings generated more issues (e.g., more consistency problems). We additionally found two moving-related factors (attentiveness and manageability) that affected cross-platform UX. Another outcome of this study was that users reported several UX design issues associated with mobile UIs operating while walking. We analysed the issues and proposed five UX design principles (reduction, aesthetic simplicity, enlargement, error prevention and icon use) for mobile UIs in moving situations. This suggests benefits in having context-aware cross-platform services. Our fourth user study sought a relationship between eye-tracking metrics and cross-platform usability problems. We user tested three cross-platform services and identified a set of usability problems. We separated the identified problems into traditional and cross-platform usability problems. Some of the cross-platform usability problems were associated with user eye-tracking patterns (ETPs). We found that inconsistency was the main source of cross-platform usability problems and we recommend some considerations for evaluators to use as indicators when predicting possible cross-platform usability problems. In our final study for this research, we conducted a survey of eight professionals specialising in UX to seek their opinions about our developed cross-platform usability assessment model and UX design framework. The model and framework were finalised, incorporating results from all of our previous user studies. Overall, UX experts assessed the model and the framework as appropriate and useful.