  This paper presents key aspects and trade-offs that designers and
Human-Computer Interaction practitioners might encounter when designing
multimodal interaction for older adults. The paper gathers literature on
multimodal interaction and assistive technology, and describes a set of design
challenges specific for older users. Building on these main design challenges,
four trade-offs in the design of multimodal technology for this target group
are presented and discussed. To highlight the relevance of the trade-offs in
the design process of multimodal technology for older adults, two of the four
reported trade-offs are illustrated with two user studies that explored mid-air
and speech-based interaction with a tablet device. The first study investigates
the design trade-offs related to redundant multimodal commands in older,
middle-aged and younger adults, whereas the second one investigates the design
choices related to the definition of a set of mid-air one-hand gestures and
voice input commands. Further reflections highlight the design trade-offs that
such considerations bring in the process, presenting an overview of the design
choices involved and of their potential consequences.
