  Human supervisors in multi-robot systems are primarily responsible for
monitoring robots, but can also be assigned with secondary tasks. These tasks
can act as interruptions and can be categorized as either intrinsic, i.e.,
being directly related to the monitoring task, or extrinsic, i.e., being
unrelated. In this paper, we investigate the impact of these two types of
interruptions through a user study ($N=39$), where participants monitor a
number of remote mobile robots while intermittently being interrupted by either
a robot fault correction task (intrinsic) or a messaging task (extrinsic). We
find that task performance of participants does not change significantly with
the interruptions but depends greatly on the number of robots. However,
interruptions result in an increase in perceived workload, and extrinsic
interruptions have a more negative effect on workload across all NASA-TLX
scales. Participants also reported switching between extrinsic interruptions
and the primary task to be more difficult compared to the intrinsic
interruption case. Statistical significance of these results is confirmed using
ANOVA and one-sample t-test. These findings suggest that when deciding task
assignment in such supervision systems, one should limit interruptions from
secondary tasks, especially extrinsic ones, in order to limit user workload.
