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Forum Ranking Diversification


Published in Learning@Scale 2017. Paper:

If you find this repo or the paper helpful, please cite us:

 author = {Northcutt, Curtis G. and Leon, Kimberly A. and Chen, Naichun},
 title = {Comment Ranking Diversification in Forum Discussions},
 booktitle = {Proceedings of the Fourth (2017) ACM Conference on Learning @ Scale},
 series = {L@S '17},
 year = {2017},
 isbn = {978-1-4503-4450-0},
 location = {Cambridge, Massachusetts, USA},
 pages = {327--330},
 numpages = {4},
 url = {},
 doi = {10.1145/3051457.3054016},
 acmid = {3054016},
 publisher = {ACM},
 address = {New York, NY, USA},
 keywords = {discussion forum, embeddings, information retrieval, online courses, ranking diversification, search},

Why diversify?

Text ranking systems (e.g. Facebook post comments, Amazon product reviews, Reddit forums) are ubiquitous, yet many suffer from a common problem. When items (e.g. responses or comments) are ranked primarily by text content and rating (e.g. like/unlike, +/-, etc.), then similar items tend to receive similar scores, often producing redundant items with similar ranking. For example, if “Great job!” is ranked first, then “Great job.” is likely to be ranked second. Moreover, higher ranking items tend to only represent the majority opinion, since there are more users in the majority group to up-vote items sharing their sentiment; thus, for systems with thousands of items in a single forum, since most users only view the highest-ranked items, most users will often only be exposed to the majority opinion.

In this paper, we develop an algorithm for forum comment ranking diversification using maximal marginal relevance (MMR) to linearly interpolate between the original item ranking (relevance) and the similarity of an item to higher-ranked items (diversity). A single parameter, λ, is used to adjust this trade-off. Item similarity is captured using the cosine similarity of tf-idf bag of words representation, where each word is embedded using a PCA+TFIDF embedding model trained on a corpora of 100,000+ edX course discussion forum responses. We apply our model to the forum discussions of an online course, MITx 6.00.1x, where capturing the diversity of responses may be of high importance to debunk misconceptions held by the majority of forum respondents and to capture the diversity of posts across thousands of learners.


Textual forum posts and replies will be obtained via web-scraping. A database for multiple MITx online edX courses will store the following columns:

  1. username
  2. comment_text
  3. comment_type (reply or post)
  4. original comment rank.

Original comment scores (number of likes) cannot be inferred via web-scraping. Instead we weakly estimate the score using the current original ranking and assuming a uniform distribution of scores from 0 to 1.

Baseline Measure

The baseline model simply ranks comments by their number of replies and upvotes.


We measure the effect on learning outcomes using a double-blind experiment with 300 tests across 3 reviewers. Reviewers are asked to answer 3 questions between two rankings, one of which has secretely been ordered by our algorithm. Questions focus on (1) diversity, (2) redundancy, (3) inclusion of the content of another comment in the forum. Cohen's cappa is used to show inter-rater reliability. For all three questions, reviewers selected our algorithm's ordering of the comments by a significant margin for all three questions.