  Clear presentation of uncertainty is an exception rather than rule in media
articles, data-driven reports, and consumer applications, despite proposed
techniques for communicating sources of uncertainty in data. This work
considers, Why do so many visualization authors choose not to visualize
uncertainty? I contribute a detailed characterization of practices,
associations, and attitudes related to uncertainty communication among
visualization authors, derived from the results of surveying 90 authors who
regularly create visualizations for others as part of their work, and
interviewing thirteen influential visualization designers. My results highlight
challenges that authors face and expose assumptions and inconsistencies in
beliefs about the role of uncertainty in visualization. In particular, a clear
contradiction arises between authors' acknowledgment of the value of depicting
uncertainty and the norm of omitting direct depiction of uncertainty. To help
explain this contradiction, I present a rhetorical model of uncertainty
omission in visualization-based communication. I also adapt a formal
statistical model of how viewers judge the strength of a signal in a
visualization to visualization-based communication, to argue that uncertainty
communication necessarily reduces degrees of freedom in viewers' statistical
inferences. I conclude with recommendations for how visualization research on
uncertainty communication could better serve practitioners' current needs and
values while deepening understanding of assumptions that reinforce uncertainty
omission.
