Ludovico Boratto

State-of-the-Art in Group Recommendation and New Approaches for Automatic Identification of Groups

Recommender systems are important tools that provide information items to users, by adapting to their characteristics and preferences. Usually items are recommended to individuals, but there are contexts in which people operate in groups. To support the recommendation process in social activities, group recommender systems were developed. Since different types of groups exist, group recommendation should adapt to them, managing heterogeneity of groups. This chapter will present a survey of the state-of-the-art in group recommendation, focusing on the type of group each system aims to. A new approach for group recommendation is also presented, able to adapt to technological constraints (e.g., bandwidth limitations), by automatically identifying groups of users with similar interests.

Group Recommendation with Automatic Identification of Users Communities

Recommender systems usually propose items to single users. However, in some domains like Mobile IPTV or Satellite Systems it might be impossible to generate a program schedule for each user, because of bandwidth limitations. A few approaches were proposed to generate group recommendations. However, these approaches take into account that groups of users already exist and no recommender system is able to detect intrinsic users communities. This paper describes an algorithm that detects groups of users whose preferences are similar and predicts recommendations for such groups. Groups of different granularities are generated through a modularity-based Community Detection algorithm, making it possible for a content provider to explore the trade off between the level of personalization of the recommendations and the number of channels. Experimental results show that the quality of group recommendations increases linearly with the number of groups created.

Using Collaborative Filtering to Overcome the Curse of Dimensionality when Clustering Users in a Group Recommender System

A characteristic of most datasets is that the number of data points is much lower than the number of dimensions (e.g., the number of movies rated by a user is much lower than the number of movies in a dataset). Dealing with high-dimensional and sparse data leads to problems in the classification process, known as curse of dimensionality. Previous researches presented approaches that produce group recommendations by clustering users in contexts where groups are not available. In the literature it is widely-known that clustering is one of the classification forms affected by the curse of dimensionality. In this paper we propose an approach to remove sparsity from a dataset before clustering users in group recommendation. This is done by using a Collaborative Filtering approach that predicts the missing data points. In such a way, it is possible to overcome the curse of dimensionality and produce better clusterings. Experimental results show that, by removing sparsity, the accuracy of the group recommendations strongly increases with respect to a system that works on sparse data.

Modeling the Preferences of a Group of Users Detected by Clustering: a Group Recommendation Case-Study

Group modeling is the process that combines multiple user models into a single model. In group recommendation, this allows to derive a group preference for each item. It is known that the strategy used to model a group has to be chosen considering the domain in which the system operates. This paper evaluates group modeling strategies in a group recommendation scenario in which groups are detected by clustering users. Once users are clustered, strategies are tested, in order to find the one that allows to get the best accuracy. Experimental results show that clustering and group modeling are strongly connected. By producing group preferences that are equally distant from the individual preferences, the modeling strategy has the same role that the centroid has when users are clustered. This previously unknown link among the two tasks is essential in order to build accurate group recommendations.

The rating prediction task in a group recommender system that automatically detects groups: architectures, algorithms, and performance evaluation

A recommender system suggests items to users by predicting what might be interesting for them. The prediction task has been highlighted in the literature as the most important one computed by a recommender system. Its role becomes even more central when a system works with groups, since the predictions might be built for each user or for the whole group. This paper presents a deep evaluation of three approaches, used for the prediction of the ratings in a group recommendation scenario in which groups are detected by clustering the users. Experimental results confirm that the approach to predict the ratings strongly influences the performance of a system and show that building predictions for each user, with respect to building predictions for a group, leads to great improvements in the accuracy of the recommendations.

A semantic approach to remove incoherent items from a user profile and improve the accuracy of a recommender system

Recommender systems usually suggest items by exploiting all the previous interactions of the users with a system (e.g., in order to decide the movies to recommend to a user, all the movies she previously purchased are considered). This canonical approach sometimes could lead to wrong results due to several factors, such as a change in user preferences over time, or the use of her account by third parties. This kind of incoherence in the user profiles defines a lower bound on the error the recommender systems may achieve when they generate suggestions for a user, an aspect known in literature as magic barrier. This paper proposes a novel dynamic coherence-based approach to define the user profile used in the recommendation process. The main aim is to identify and remove, from the previously evaluated items, those not semantically adherent to the others, in order to make a user profile as close as possible to the user’s real preferences, solving the aforementioned problems. Moreover, reshaping the user profile in such a way leads to great advantages in terms of computational complexity, since the number of items considered during the recommendation process is highly reduced. The performed experiments show the effectiveness of our approach to remove the incoherent items from a user profile, increasing the recommendation accuracy.

Design and architecture of a friend recommender system in the social bookmarking domain

Social media systems are becoming more and more popular nowadays. In order to face the overload in the amount of users and content available in these systems, social recommender systems have been developed and are largely studied in the literature. A form of social media, known as social bookmarking system, allows to share bookmarks in a social network. A user adds as a friend or follows another user and receives updates on the bookmarks added by that user. However, no approach in the literature proposes friend recommender systems in the social bookmarking domain. In this paper, we present an analysis of the state-of-the-art on user recommendation in social environments and of the structure of a social bookmarking system, in order to derive a design and an architecture of a friend recommender system in the social bookmarking domain. This study can be useful for any future research in this area, by highlighting the aspects that characterize this domain and the features that this type of recommender system has to offer.

Behavioral data mining to produce novel and serendipitous friend recommendations in a social bookmarking system

In the last few years, social media systems have experienced a fast growth. The amount of content shared in these systems increases fast, leading users to face the well known “interaction overload” problem, i.e., they are overwhelmed by content, so it becomes difficult to come across interesting items. To overcome this problem, social recommender systems have been recently designed and developed in order to filter content and recommend to users only interesting items. This type of filtering is usually affected by the “over-specialization” problem, which is related to recommendations that are too similar to the items already considered by the users. This paper proposes a friend recommender system that operates in the social bookmarking application domain and is based on behavioral data mining, i.e., on the exploitation of the users activity in a social bookmarking system. Experimental results show how this type of mining is able to produce accurate friend recommendations, allowing users to get to know bookmarked resources that are both novel and serendipitous. Using this approach, the impact of the “interaction overload” and the “over-specialization” problems is strongly reduced.

Using neural word embeddings to model user behavior and detect user segments

Modeling user behavior to detect segments of users to target and to whom address ads (behavioral targeting) is a problem widely-studied in the literature. Various sources of data are mined and modeled in order to detect these segments, such as the queries issued by the users. In this paper we first show the need for a user segmentation system to employ reliable user preferences, since nearly half of the times users reformulate their queries in order to satisfy their information need. Then we propose a method that analyzes the description of the items positively evaluated by the users and extracts a vector representation of the words in these descriptions (word embeddings). Since it is widely-known that users tend to choose items of the same categories, our approach is designed to avoid the so-called preference stability, which would associate the users to trivial segments. Moreover, we make sure that the interpretability of the generated segments is a characteristic offered to the advertisers who will use them. We performed different sets of experiments on a large real-world dataset, which validated our approach and showed its capability to produce effective segments.

Using social media to characterize urban mobility patterns: State-of-the-art survey and case-study

Abstract The knowledge of the urban mobility is a crucial aspect for city planners and administrators. The huge amount of geo-spatial data, generated by the combination of social media systems and the wide use of smart devices, is creating new challenges and opportunities to satisfy this thirst of knowledge. In this work, we explore how social media data can be used to infer knowledge about urban dynamics and mobility patterns in a urban area. Specifically, in order to highlight the main advantages, limitations, and open issues, we focus on mobility patterns by presenting a survey of the state of the art and a case-study based on the city of Barcelona.