  The Internet has made it easier for social scientists to study human behavior
by analyzing their interactions on social media platforms. Many of these
platforms characterize conversations among users via threads, which induce a
tree-like structure. The structural properties of these discussion trees, such
as their width, depth, and size, can be used to make inferences regarding user
discussion patterns and conversation dynamics. In this paper, we seek to
understand the structure of these online discussions on Reddit. We characterize
the structure of these discussions via a set of global and local
discussion-tree properties. The global features constitute information
regarding the community/subreddit of a given post, whereas the local features
are comprised of the properties of the post itself. We perform various
statistical analyses on a year's worth of Reddit data containing a quarter of a
million posts and several million comments. These analyses allow us to tease
apart the relative contribution of a discussion post's global and local
properties and characterize the importance of specific individual features in
determining the discussions' structural patterns. Our results indicate that
both local and global features explain a significant amount of structural
variation. Local features are collectively more important as they explain
significantly more variation in the discussion trees' structural properties
than global features. However, there is significant heterogeneity in the impact
of the various features. Several global features, e.g., the topic, age,
popularity, and the redundancy of content in a subreddit, also play a crucial
role in understanding the specific properties of discussion trees.
