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% score, 25% 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.
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