## 7 Limitations  
One limitation of our work is that our experimental design focused on discrete events of interruption modes and distinctive task types. Though this is inevitable and intentional to clarify the collected data, natural and mixed transitions between these discrete levels or task types are required to reflect actual tasking environments. In addition, though interruptions may also be incurred internally [65], we considered only external interruptions. For future work, we plan on introducing more realistic interruption processes and more diverse task examples. However, we see this model as a first step toward the comprehensive estimation of interruption cost through pupil diameter changes, task completion time and accuracy, and a subjective workload measure.  
The lack of ecological validity in this study's tasks is another limitation. The tasks used in this study were designed based on contrived and laboratory-oriented tasks that are far from actual, authentic tasks in a real setting, and the tasks used lack generalizability and fidelity. Furthermore, suspended primary tasks were occluded by interrupting tasks in all interruption modes in this study. This also results in an ecological validity issue because total occlusions are not frequent in actual task settings. Such differences in task type affect not only experiment and study design, but also the disreputability of interruption. Arbitrary, contrived tasks can manipulate design factors such as task difficulty and interruption moments and durations, whereas actual tasks in natural settings are easily supported by various post-interruption tools and artifacts, which guide one back into the task in other ways. Several interruption studies were carried out with actual authentic tasks including the comparison of interruption modes in a manufacturing process [66] and the investigation of resumption lags in interrupted critical care tasks in the healthcare domain [67]; however, more interruption studies are still needed to understand how to minimize the negative effects of interruptions.  
Another limitation is that the study tested only three out of four interruption coordination modes, as defined by [PERSON]. This study ruled out the mediated mode because we mainly focused on direct interaction between users and interruption, where the mediated mode depends greatly on the agent function to determine when and how interruptions interfere. However, four interruption coordination modes reflect well the interruption patterns in user interface design guidelines [21], and we need to include the mediated mode in a more general application for the human-computer interaction (HCI) domain.  
Individual differences and narrow participant distribution are other considerations in the estimation of interruption cost. Specifically, individual differences result in a widely varying degree of problem state and focus of attention, which are critical in information processing [68]. Furthermore, the interruption cost depends on the duration, the moments, and the complexity of interruptions. In addition, the experiments were limited to college students who major in engineering. A broader range of participants needs to be recruited in future studies. Finally, a number of environmental variables, such as ambient lighting, as well as task modality and difficulty, will change continuously in time and value.