  We present an observational study to compare co-located and situated
real-time visualizations in basketball free-throw training. Our goal is to
understand the advantages and concerns of applying immersive visualization to
real-world skill-based sports training and to provide insights for designing AR
sports training systems. We design both a situated 3D visualization on a
head-mounted display and a 2D visualization on a co-located display to provide
immediate visual feedback on a player's shot performance. Using a
within-subject study design with experienced basketball shooters, we
characterize user goals, report on qualitative training experiences, and
compare the quantitative training results. Our results show that real-time
visual feedback helps athletes refine subsequent shots. Shooters in our study
achieve greater angle consistency with our visual feedback. Furthermore, AR
visualization promotes an increased focus on body form in athletes. Finally, we
present suggestions for the design of future sports AR studies.
