Thispaperconsidersextractivesummarisationinacomparative setting: given two or more document groups (e.g., separated by publication time), the goal is to select a small number of documents that are representative of each group, and also maximally distinguishable from other groups. We formulate a set of new objective functions for this problem that connect recent literature on document summarisation, interpretable machine learning, and data subset selection. In particular, by casting the problem as a binary classification amongst different groups, we derive objectives based on the notion of maximum mean discrepancy, as well as a simple yet effective gradient-based optimisation strategy. Our new formulation allows scalable evaluations of comparative summarisation as a classification task, both automatically and via crowd-sourcing. To this end, we evaluate comparative summarisation methods on a newly curated collection of controversial news topics over 13months.Weobserve thatgradient-based optimisationoutperforms discrete and baseline approaches in 15 out of 24 different automatic evaluation settings. In crowd-sourced evaluations, summaries from gradient optimisation elicit 7% more accurate classification from human workers than discrete optimisation. Our result contrasts with recent literature on submodular data subset selection that favours discrete optimisation. We posit that our formulation of comparative summarisation will prove useful in a diverse range of use cases such as comparing content sources, authors, related topics, or distinct view points.

The goal of this thesis is to develop techniques for comparative summarisation of multimodal document collections. Comparative summarisation is extractive summarisation in comparative settings, where documents form two or more groups, e.g. articles on the same topic but from different sources. Comparative summarisation involves, not only, selecting representative and diverse samples within groups, but also samples that highlight commonalities and differences between the groups. We posit that comparative summarisation is a fruitful problem for diverse use cases, such as comparing content over time, authors, or distinct view points. We formulate the problem of comparative summarisation by reducing it to binary classification problem and define objectives to incorporate representativeness, diversity and comparativeness. We design new automatic and crowd-sourced evaluation protocols for summarisation evaluation that scales much better than the evaluations requiring manually created ground truth summaries. We show the efficacy of the approach in a newly curated datasets of controversial news topics. We plan to develop new collection comparison methods for multimodal document collections.

We propose KGdiff, a new interactive visualization tool for social media content focusing on entities and relationships. The core component is a layout algorithm that highlights the differences between two graphs. We apply this algorithm on knowledge graphs consisting of named entities and their relations extracted from text streams over different time periods. The visualization system provides additional information such as the volume and frequency ranking of entities and allows users to select which parts of the graph to visualize interactively. On Twitter and news article collections, KGdiff allows users to compare different data subsets. Results of such comparisons often reveal topical or geographical changes in a discussion. More broadly, graph differences are useful for a wide range of relational data comparison tasks, such as comparing social interaction graphs, identifying changes in user behavior, or discovering differences in graphs from distinct sources, geography, or political stance.