## 6 Discussion
### Evaluation of Interruption Cost by Interruption Cordination Mode and Task Type  
Cognitive workload data measured by pupil diameters and the NASA TLX method, as well as task performance data including task completion time and task accuracy. Indicated that the interruption costs varied with interruption mode and task set. Two trends are visible in the data analysis. Firstly, the average interruption costs clearly vary with interruption mode. In user-oriented costs, both physiological and subjective workload measures indicated that the immediate mode showed the highest cost and the negotiated mode demonstrated the lowest cost. Interestingly, while the scheduled mode and the negotiated mode were significantly different in pupil diameter variation, they were not distinctive in subjective cognitive workload measures (NASA TLX scores). This may be due to limitations in the self-reported survey-based approach. Due to the feature of after-event data collection, participants hardly recalled and recognized workload variations in their perception of entire task experiences. Task-oriented costs showed different patterns. The negotiated mode scored the lowest error rates, but the scheduled mode recorded shorter task completion times than the immediate mode. Because task accuracy counts the errors in primary tasks, participants in the negotiated mode could maintain full attention during the task, which resulted in the fewest number of errors in both cognitive and skill tasks. This may be due to the participants' ability to control the initiation of interruptions in the negotiated mode. This pattern is well matched with the lowest cognitive workload by the physiological measure. For task completion time, the time difference in the scheduled and the immediate modes can be explained by the roles of interruption lag [30]. Participants were fully aware of the onset timing of interrupting peripheral tasks in the scheduled mode, whereas participants were not ready to transfer to peripheral tasks when an interruption occurred in the immediate mode. This resulted in shorter interruption lag and total task completion time in the scheduled mode than in the immediate mode. Because the interruption coordination modes can be interpreted as guides for how to deliver interruptions, interruption lag depends more on the mode than the resumption lag required to resume suspended primary tasks. In the negotiated mode, participants did not experience any time pressure on interruption and recorded the longest time. Secondly, the interruption costs also highly depended on the different type of primary task. Generally, compared to skill/skill sets, cognitive/cognitive sets showed the highest workload and the longest task completion time. The cost by task type depended on the performance of the primary task. Cognitive primary task sets (cognitive/cognitive and cognitive/skill tasksets) had higher workload and worse task performance than skill primary task sets (skill/cognitive and skill/skill task sets). However, the cost by task type was affected by interruption coordination modes as well. In particular, in the negotiated mode, skill/skill task sets demonstrated the highest task accuracy, and similar levels of subjective workload were measured across all task sets. For task performance, as [PERSON] and [PERSON] [59] reported, cognitive task sets (cognitive/cognitive and cognitive/skill task sets) need more time for task completion, and skill task sets (skill/cognitive and skill/skill task sets) generate more errors. The memory for goals theory supports the explanation of the resumption process, where the most active goal in memory is pursued [23; 30]. The goals of interrupted (suspended) tasks decay over time and lose activation. The base-level activation (task history) and associated problem state (current context) enable overcoming decay over time and keep relevant goals to be resumed. Generally, longer resumption lags make a goal less active, and therefore, more difficult to be resumed [61; 62].