  Persuading people to change their opinions is a common practice in online
discussion forums on topics ranging from political campaigns to relationship
consultation. Enhancing people's ability to write persuasive arguments could
not only practice their critical thinking and reasoning but also contribute to
the effectiveness and civility in online communication. It is, however, not an
easy task in online discussion settings where written words are the primary
communication channel. In this paper, we derived four design goals for a tool
that helps users improve the persuasiveness of arguments in online discussions
through a survey with 123 online forum users and interviews with five debating
experts. To satisfy these design goals, we analyzed and built a labeled dataset
of fine-grained persuasive strategies (i.e., logos, pathos, ethos, and
evidence) in 164 arguments with high ratings on persuasiveness from
ChangeMyView, a popular online discussion forum. We then designed an
interactive visual system, Persua, which provides example-based guidance on
persuasive strategies to enhance the persuasiveness of arguments. In
particular, the system constructs portfolios of arguments based on different
persuasive strategies applied to a given discussion topic. It then presents
concrete examples based on the difference between the portfolios of user input
and high-quality arguments in the dataset. A between-subjects study shows
suggestive evidence that Persua encourages users to submit more times for
feedback and helps users improve more on the persuasiveness of their arguments
than a baseline system. Finally, a set of design considerations was summarized
to guide future intelligent systems that improve the persuasiveness in text.
