Comment Ranking Diversification in Forum Discussions

Viewing consumption of discussion forums with hundreds or more comments depends on ranking because most users only view top-ranked comments. When comments are ranked by an ordered score (e.g. number of replies or up-votes) without adjusting for semantic similarity of near-ranked comments, top-ranked comments are more likely to emphasize the majority opinion and incur redundancy. In this paper, we propose a top K comment diversification re-ranking model using Maximal Marginal Relevance (MMR) and evaluate its impact in three categories: (1) semantic diversity, (2) inclusion of the semantics of lower-ranked comments, and (3) redundancy, within the context of a HarvardX course discussion forum. We conducted a double-blind, small-scale evaluation experiment requiring subjects to select between the top 5 comments of a diversified ranking and a baseline ranking ordered by score. For three subjects, across 100 trials, subjects selected the diversified (75 diversification) ranking as significantly (1) more diverse, (2) more inclusive, and (3) less redundant. Within each category, inter-rater reliability showed moderate consistency, with typical Cohen-Kappa scores near 0.2. Our findings suggest that our model improves (1) diversification, (2) inclusion, and (3) redundancy, among top K ranked comments in online discussion forums.