  Mixed Reality (MR) is gaining prominence in manual task skill learning due to
its in-situ, embodied, and immersive experience. To teach manual tasks, current
methodologies break the task into hierarchies (tasks into subtasks) and
visualize the current subtask and future in terms of causality. Existing
psychology literature also shows that humans learn tasks by breaking them into
hierarchies. In order to understand the design space of information visualized
to the learner for better task understanding, we conducted a user study with 48
users. The study was conducted using a complex assembly task, which involves
learning of both actions and tool usage. We aim to explore the effect of
visualization of causality in the hierarchy for manual task learning in MR by
four options: no causality, event level causality, interaction level causality,
and gesture level causality. The results show that the user understands and
performs best when all the level of causality is shown to the user. Based on
the results, we further provide design recommendations and in-depth discussions
for future manual task learning systems.
