Denis Helic

Denis Helic
Graz University of Technology | TU Graz · Knowledge Technologies Institute

About

168
Publications
22,639
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1,419
Citations

Publications

Publications (168)
Preprint
We present evidence for the existence and effectiveness of adversarial attacks on graph neural networks (GNNs) that aim to degrade fairness. These attacks can disadvantage a particular subgroup of nodes in GNN-based node classification, where nodes of the underlying network have sensitive attributes, such as race or gender. We conduct qualitative a...
Article
Full-text available
In order to protect gamblers, gambling operators have introduced a wide range of responsible gambling (RG) tools. Mandatory play breaks (i.e., forced termination of a gambling session) and personalized feedback about the gambling expenditure are two RG tools that are frequently used. While the motivation behind mandatory play breaks is simple (i.e....
Article
Full-text available
Protecting gamblers from problematic gambling behavior is a major concern for clini-cians, researchers, and gambling regulators. Most gambling operators offer a range of so-called responsible gambling tools to help players better understand and control their gambling behavior. One such tool is voluntary self-exclusion, which allows players to block...
Article
Full-text available
Monitoring hygiene and motivation factors from Herzberg’s Two-Factor Theory is a popular way of understanding the influential aspects for employee satisfaction and motivation. The increased availability of employee feedback comprised in online employer reviews yields a promising data source to learn more about these influential factors and the theo...
Article
Full-text available
Complex systems, abstractly represented as networks, are ubiquitous in everyday life. Analyzing and understanding these systems requires, among others, tools for community detection. As no single best community detection algorithm can exist, robustness across a wide variety of problem settings is desirable. In this work, we present Synwalk, a rando...
Article
Full-text available
Wikipedia, the largest encyclopedia ever created, is a global initiative driven by volunteer contributions. When the COVID-19 pandemic broke out and mobility restrictions ensued across the globe, it was unclear whether contributions to Wikipedia would decrease in the face of the pandemic, or whether volunteers would withstand the added stress and i...
Article
Full-text available
Graph Neural Networks (GNNs) are effective in many applications. Still, there is a limited understanding of the effect of common graph structures on the learning process of GNNs. To fill this gap, we study the impact of community structure and homophily on the performance of GNNs in semi-supervised node classification on graphs. Our methodology con...
Conference Paper
Full-text available
Wikipedia is one of the most popular sites on the web and an essential knowledge source for millions of web users. As this online encyclopedia significantly shapes our image of the world, its neutrality is of utmost importance. However, past studies demonstrated that Wikipedia is frequently unbalanced or biased in various subject areas. In this wor...
Preprint
Full-text available
Recent work has shown that graph neural networks (GNNs) are vulnerable to adversarial attacks on graph data. Common attack approaches are typically informed, i.e. they have access to information about node attributes such as labels and feature vectors. In this work, we study adversarial attacks that are uninformed, where an attacker only has access...
Article
Full-text available
Previous research suggests that the presence of other gamblers in a gambling venue intensifies individual gambling behavior. To study such potential social facilitation among gamblers, we conduct a large-scale analysis of more than one million gambling sessions using player tracking data of the Norwegian gambling operator Norsk Tipping. In particul...
Preprint
Full-text available
In recent years, Radio Frequency Identification (RFID) technology has been applied to improve numerous processes, such as inventory management in retail stores. However, automatic localization of RFID-tagged goods in stores is still a challenging problem. To address this issue, we equip fixtures (e.g., shelves) with reference tags and use data we c...
Preprint
Full-text available
Tagging facilitates information retrieval in social media and other online communities by allowing users to organize and describe online content. Researchers found that the efficiency of tagging systems steadily decreases over time, because tags become less precise in identifying specific documents, i.e., they lose their descriptiveness. However, p...
Preprint
Full-text available
As a tool for capturing irregular temporal dependencies (rather than resorting to binning temporal observations to construct time series), Hawkes processes with exponential decay have seen widespread adoption across many application domains, such as predicting the occurrence time of the next earthquake or stock market spike. However, practical appl...
Preprint
Full-text available
Wikipedia, the largest encyclopedia ever created, is a global initiative driven by volunteer contributions. When the COVID-19 pandemic broke out and mobility restrictions ensued across the globe, it was unclear whether Wikipedia volunteers would become less active in the face of the pandemic, or whether they would rise to meet the increased demand...
Preprint
Complex systems, abstractly represented as networks, are ubiquitous in everyday life. Analyzing and understanding these systems requires, among others, tools for community detection. As no single best community detection algorithm can exist, robustness across a wide variety of problem settings is desirable. In this work, we present Synwalk, a rando...
Chapter
Graph Neural Networks (GNNs) are effective in many applications. Still, there is a limited understanding of the effect of common graph structures on the learning process of GNNs. In this work, we systematically study the impact of community structure on the performance of GNNs in semi-supervised node classification on graphs. Following an ablation...
Preprint
Full-text available
Graph Neural Networks (GNNs) are effective in many applications. Still, there is a limited understanding of the effect of common graph structures on the learning process of GNNs. In this work, we systematically study the impact of community structure on the performance of GNNs in semi-supervised node classification on graphs. Following an ablation...
Chapter
Full-text available
Employee satisfaction impacts the efficiency of businesses as well as the lives of employees spending substantial amounts of their time at work. As such, employee satisfaction attracts a lot of attention from researchers. In particular, a lot of effort has been previously devoted to the question of how to positively influence employee satisfaction,...
Article
Full-text available
Currently, the relation between edit behavior, link structure, and article quality is not well-understood in our community, notwithstanding that this relationship may facilitate editing processes and content quality on Wikipedia. To shed light on this complex relation, we classify article edits and perform an in-depth analysis of editing sequences...
Chapter
Full-text available
In this work, we study the utility of graph embeddings to generate latent user representations for trust-based collaborative filtering. In a cold-start setting, on three publicly available datasets, we evaluate approaches from four method families: (i) factorization-based, (ii) random walk-based, (iii) deep learning-based, and (iv) the Large-scale...
Article
Full-text available
Abstract Spectator periodicals contributed to spreading the ideas of the Age of Enlightenment, a turning point in human history and the foundation of our modern societies. In this work, we study the spirit and atmosphere captured in the spectator periodicals about important social issues from the 18th century by analyzing text sentiment of those pe...
Preprint
Full-text available
In this work, we study the utility of graph embeddings to generate latent user representations for trust-based collaborative filtering. In a cold-start setting, on three publicly available datasets, we evaluate approaches from four method families: (i) factorization-based, (ii) random walk-based, (iii) deep learning-based, and (iv) the Large-scale...
Chapter
Full-text available
When editing articles on Wikipedia, arguments between editors frequently occur. These conflicts occasionally lead to destructive behavior and diminish article quality. Currently, the relation between editing behavior, link structure, and article quality is not well-understood in our community, notwithstanding that this relation may facilitate editi...
Chapter
Spectator journals published during the Age of Enlightenment served to enhance morality of readers and focused on a plethora of topics, such as the image of women or men, politics and religion. Although spectator journals have been studied extensively, little is known about the sentiment that they express. In this paper, we analyze text sentiment o...
Book
This book constitutes the proceedings of the 25th International Symposium on Foundations of Intelligent Systems, ISMIS 2020, held in Graz, Austria, in October 2020. The conference was held virtually due to the COVID-19 pandemic. The 35 full and 8 short papers presented in this volume were carefully reviewed and selected from 79 submissions. Include...
Article
As video game press ("experts") and casual gamers ("amateurs") have different motivations when writing video game reviews, discrepancies in their reviews may arise. To study such potential discrepancies, we conduct a large-scale investigation of more than 1 million reviews on the Metacritic review platform. In particular, we assess the existence an...
Chapter
Numerous collaboration websites struggle to achieve self-sustainability—a level of user activity preventing a transition to a non-active state. We know only a little about the factors which separate sustainable and successful collaboration websites from those that are inactive or have a declining activity. We argue that modeling and understanding v...
Conference Paper
In this paper, we quantify the impact of self- and cross-excitation on the temporal development of user activity in Stack Exchange Question & Answer (Q&A) communities. We study differences in user excitation between growing and declining Stack Exchange communities, and between those dedicated to STEM and humanities topics by leveraging Hawkes proce...
Conference Paper
Recommender systems have become omni-present tools that are used by a wide variety of users in everyday life tasks, such as finding products in Web stores or online movie streaming portals. However, in situations where users already have an idea of what they are looking for (e.g., 'The Lord of the Rings', but in space with a dark vibe), most tradit...
Article
Millions of users on the Internet discuss a variety of topics on Question-and-Answer (Q8A) instances. However, not all instances and topics receive the same amount of attention, as some thrive and achieve self-sustaining levels of activity, while others fail to attract users and either never grow beyond being a small niche community or become inact...
Conference Paper
Recently, networks of user interactions in online systems gained a lot of interest from our research community. Such networks are characterized by complex bursty patterns of human user behavior. A lot of models for such networks are based on the activity-driven time-varying network framework, which was introduced in an effort to model human interac...
Chapter
Allowing users to organize content by tagging resources in webbased systems has led to the emergence of the so-called SocialWeb. Tags turned out to be helpful not only for giving recommendations and improving search in social tagging systems but also for enhancing information access by navigating. In this chapter, we will cover much of the pioneer...
Article
Full-text available
Background In this paper, we study the process of opinion dynamics and consensus building in online collaboration systems, in which users interact with each other following their common interests and their social profiles. Specifically, we are interested in how users similarity and their social status in the community, as well as the interplay of t...
Conference Paper
Full-text available
One of the major challenges in the field of ambulant stress detection lies in the model validation. Commonly, different types of questionnaires are used to record perceived stress levels. These only capture stress levels at discrete moments in time and are prone to subjective inaccuracies. Although, many studies have already reported such issues, a...
Article
Massive Open Online Courses (MOOCs) are one of the fastest growing and most popular phenomena in e-learning. Universities around the world continue to invest to create and maintain these online courses. Reuse of material from previous courses is a shared practice that helps to reduce production costs and enhance future offerings. However, such re-r...
Conference Paper
Full-text available
In this paper we assess the semantic stability of Wikipedia by investigating the dynamics of Wikipedia articles’ revisions over time. In a semantically stable system, articles are infrequently edited, whereas in unstable systems, article content changes more frequently. In other words, in a stable system, the Wikipedia community has reached consens...
Conference Paper
Recommendations are increasingly used to support and enable discovery, browsing and exploration of large item collections, especially when no clear classification of items exists. Yet, the suitability of a recommendation algorithm to support these use cases cannot be comprehensively evaluated by any evaluation measures proposed so far. In this pape...
Article
Full-text available
Recommendations are increasingly used to support and enable discovery, browsing, and exploration of items. This is especially true for entertainment platforms such as Netflix or YouTube, where frequently, no clear categorization of items exists. Yet, the suitability of a recommendation algorithm to support these use cases cannot be comprehensively...
Article
Millions of users on the Internet discuss a variety of topics on Question and Answer (Q&A) instances. However, not all instances and topics receive the same amount of attention, as some thrive and achieve self-sustaining levels of activity while others fail to attract users and either never grow beyond being a small niche community or become inacti...
Conference Paper
In recent years, technology enhanced learning platforms became widely accessible. In particular, the number of Massive Open Online Courses (MOOCs) has—and still is—constantly growing. This widespread adoption of MOOCs triggered the development of specialized solutions, that emphasize or enhance various aspects of traditional MOOCs. Despite this sig...
Article
When users interact with the Web today, they leave sequential digital trails on a massive scale. Examples of such human trails include Web navigation, sequences of online restaurant reviews, or online music play lists. Understanding the factors that drive the production of these trails can be useful, for example, for improving underlying network st...
Article
Full-text available
Ontologies in the biomedical domain are numerous, highly specialized and very expensive to develop. Thus, a crucial prerequisite for ontology adoption and reuse is effective support for exploring and finding existing ontologies. Towards that goal, the National Center for Biomedical Ontology (NCBO) has developed BioPortal---an online repository cont...
Conference Paper
Ontologies in the biomedical domain are numerous, highly specialized and very expensive to develop. Thus, a crucial prerequisite for ontology adoption and reuse is effective support for exploring and finding existing ontologies. Towards that goal, the National Center for Biomedical Ontology (NCBO) has developed BioPortal---an online repository cont...
Conference Paper
Modeling activity in online collaboration websites, such as StackExchange Question and Answering portals, is becoming increasingly important, as the success of these websites critically depends on the content contributed by its users. In this paper, we represent user activity as time series and perform an initial analysis of these time series to ob...
Conference Paper
When new users join social networking websites, they often form collaboration ties with existing users, which in turn may result in some level of activity on the site. However, for various reasons, new users often fail to create such ties and their contributions to the system's overall activity remain insignificant. For example, on Question and Ans...
Article
The 28th 2017 ACM international conference on Hypertext and Social Media will be held in Prague, Czech Republic, from July 4 to 7. This newsletter article briefly introduces the conference and its venue. We hope to meet you all at Hypertext 2017! https://ht.acm.org/ht2017/
Article
Full-text available
In this paper, we analyze the influence of social status on opinion dynamics and consensus building in collaboration networks. To that end, we simulate the diffusion of opinions in empirical networks and take into account both the network structure and the individual differences of people reflected through their social status. For our simulations,...
Conference Paper
Wikipedia supports its users to reach a wide variety of goals: looking up facts, researching a topic, making an edit or simply browsing to pass time. Some of these goals, such as the lookup of facts, can be effectively supported by search functions. However, for other use cases such as researching an unfamiliar topic, users need to rely on the link...
Conference Paper
We present a new method for detecting interpretable subgroups with exceptional transition behavior in sequential data. Identifying such patterns has many potential applications, e.g., for studying human mobility or analyzing the behavior of internet users. To tackle this task, we employ exceptional model mining, which is a general approach for iden...
Article
Full-text available
In this work we study how people navigate the information network of Wikipedia and investigate (i) free-form navigation by studying all clicks within the English Wikipedia over an entire month and (ii) goal-directed Wikipedia navigation by analyzing wikigames, where users are challenged to retrieve articles by following links. To study how the orga...
Conference Paper
Websites have an inherent interest in steering user navigation in order to, for example, increase sales of specific products or categories, or to guide users towards specific information. In general, website administrators can use the following two strategies to influence their visitors' navigation behavior. First, they can introduce click biases t...
Conference Paper
Understanding human movement trajectories represents an important problem that has implications for a range of societal challenges such as city planning and evolution, public transport or crime. In this paper, we focus on geo-temporal photo trails from four different cities (Berlin, London, Los Angeles, New York) derived from Flickr that are produc...