Siamese neural networks in recommendation

Recommender systems are widely adopted as an increasing research and development area, since they provide users with diverse and useful information tailored to their needs. Several strategies have been proposed, and in most of them some concept of similarity is used as a core part of the approach, either between items or between users. At the same time, Siamese Neural Networks are being used to capture the similarity of items in the image domain, as they are defined as a subtype of Artificial Neural Networks built with (at least two) identical networks that share their weights. In this review, we study the proposals done in the intersection of these two fields, that is, how Siamese Networks are being used for recommendation. We propose a classification that considers different recommendation problems and algorithmic approaches. Some research directions are pointed out to encourage future research. To the best of our knowledge, this paper is the first comprehensive survey that focuses on the usage of Siamese Neural Networks for Recommender SystemsThis work has been funded by the Ministerio de Ciencia e Innovación (reference PID2019-108965GB-I00). The authors thank the reviewers for their thoughtful comments and suggestion

Bias characterization, assessment, and mitigation in location-based recommender systems

Location-Based Social Networks stimulated the rise of services such as Location-based Recommender Systems. These systems suggest to users points of interest (or venues) to visit when they arrive in a specific city or region. These recommendations impact various stakeholders in society, like the users who receive the recommendations and venue owners. Hence, if a recommender generates biased or polarized results, this affects in tangible ways both the experience of the users and the providers’ activities. In this paper, we focus on four forms of polarization, namely venue popularity, category popularity, venue exposure, and geographical distance. We characterize them on different families of recommendation algorithms when using a realistic (temporal-aware) offline evaluation methodology while assessing their existence. Besides, we propose two automatic approaches to mitigate those biases. Experimental results on real-world data show that these approaches are able to jointly improve the recommendation effectiveness, while alleviating these multiple polarizationsOpen Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. This work has been funded by the Ministerio de Ciencia e Innovación (reference PID2019-108965GB-I00) and by the European Social Fund (ESF), within the 2017 call for predoctoral contract

Recommender systems fairness evaluation via generalized cross entropy

Fairness in recommender systems has been considered with respect to sensitive attributes of users (e.g., gender, race) or items (e.g., revenue in a multistakeholder setting). Regardless, the concept has been commonly interpreted as some form of equality – i.e., the degree to which the system is meeting the information needs of all its users in an equal sense. In this paper, we argue that fairness in recommender systems does not necessarily imply equality, but instead it should consider a distribution of resources based on merits and needs.We present a probabilistic framework based ongeneralized cross entropy to evaluate fairness of recommender systems under this perspective, wherewe showthat the proposed framework is flexible and explanatory by allowing to incorporate domain knowledge (through an ideal fair distribution) that can help to understand which item or user aspects a recommendation algorithm is over- or under-representing. Results on two real-world datasets show the merits of the proposed evaluation framework both in terms of user and item fairnessThis work was supported in part by the Center for Intelligent Information Retrieval and in part by project TIN2016-80630-P (MINECO

A flexible framework for evaluating user and item fairness in recommender systems

This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s11257-020-09285-1One common characteristic of research works focused on fairness evaluation (in machine learning) is that they call for some form of parity (equality) either in treatment—meaning they ignore the information about users’ memberships in protected classes during training—or in impact—by enforcing proportional beneficial outcomes to users in different protected classes. In the recommender systems community, fairness has been studied with respect to both users’ and items’ memberships in protected classes defined by some sensitive attributes (e.g., gender or race for users, revenue in a multi-stakeholder setting for items). Again here, the concept has been commonly interpreted as some form of equality—i.e., the degree to which the system is meeting the information needs of all its users in an equal sense. In this work, we propose a probabilistic framework based on generalized cross entropy (GCE) to measure fairness of a given recommendation model. The framework comes with a suite of advantages: first, it allows the system designer to define and measure fairness for both users and items and can be applied to any classification task; second, it can incorporate various notions of fairness as it does not rely on specific and predefined probability distributions and they can be defined at design time; finally, in its design it uses a gain factor, which can be flexibly defined to contemplate different accuracy-related metrics to measure fairness upon decision-support metrics (e.g., precision, recall) or rank-based measures (e.g., NDCG, MAP). An experimental evaluation on four real-world datasets shows the nuances captured by our proposed metric regarding fairness on different user and item attributes, where nearest-neighbor recommenders tend to obtain good results under equality constraints. We observed that when the users are clustered based on both their interaction with the system and other sensitive attributes, such as age or gender, algorithms with similar performance values get different behaviors with respect to user fairness due to the different way they process data for each user clusterThe authors thank the reviewers for their thoughtful comments and suggestions. This work was supported in part by the Ministerio de Ciencia, Innovacion y Universidades (Reference: 123496 Y. Deldjoo et al. PID2019-108965GB-I00) and in part by the Center for Intelligent Information Retrieval. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the sponsor

How to Perform Reproducible Experiments in the ELLIOT Recommendation Framework: Data Processing, Model Selection, and Performance Evaluation

Recommender Systems have shown to be an efective way to alleviate the over-choice problem and provide accurate and tailored recommendations. However, the impressive number of proposed recommendation algorithms, splitting strategies, evaluation protocols, metrics, and tasks, has made rigorous experimental evaluation particularly challenging. ELLIOT is a comprehensive recommendation framework that aims to run and reproduce an entire experimental pipeline by processing a simple confguration fle. The framework loads, flters, and splits the data considering a vast set of strategies. Then, it optimizes hyperparameters for several recommendation algorithms, selects the best models, compares them with the baselines, computes metrics spanning from accuracy to beyond-accuracy, bias, and fairness, and conducts statistical analysis. The aim is to provide researchers a tool to ease all the experimental evaluation phases (and make them reproducible), from data reading to results collection. ELLIOT is freely available on GitHub at https://github.com/sisinflab/ellio

Analysis of co-movement pattern mining methods for recommendation

Location-Based Social Networks allow users to share the Pointsof-Interest they visit, hence creating trajectories throughout their usual lives – even though they are also used by tourists to explore a city. There exist several algorithms in the trajectory pattern mining area able to discover and exploit interesting patterns from trajectory data, such as which objects tend to move together (co-movement), however, to the best of our knowledge, they have not been used with data coming from that type of systems. In this work, we analyse the extent to which these techniques can be applied to that type of data and under which circumstances they might be usefu

Exploiting recommendation confidence in decision-aware recommender systems

The main goal of a Recommender System is to suggest relevant items to users, although other utility dimensions – such as diversity, novelty, confidence, possibility of providing explanations – are often considered. In this work, we investigate about confidence but from the perspective of the system: what is the confidence a system has on its own recommendations; more specifically, we focus on different methods to embed awareness into the recommendation algorithms about deciding whether an item should be suggested. Sometimes it is better not to recommend than fail because failure can decrease user confidence in the system. In this way, we hypothesise the system should only show the more reliable suggestions, hence, increasing the performance of such recommendations, at the expense of, presumably, reducing the number of potential recommendations. Different from other works in the literature, our approaches do not exploit or analyse the input data but intrinsic aspects of the recommendation algorithms or of the components used during prediction are considered. We propose a taxonomy of techniques that can be applied to some families of recommender systems allowing to include mechanisms to decide if a recommendation should be generated. In particular, we exploit the uncertainty in the prediction score for a probabilistic matrix factorisation algorithm and the family of nearest-neighbour algorithms, the support of the prediction score for nearest-neighbour algorithms, and a method independent of the algorithm. We study how the performance of a recommendation algorithm evolves when it decides not to recommend in some situations. If the decision of avoiding a recommendation is sensible – i.e., not random but related to the information available to the system about the target user or item –, the performance is expected to improve at the expense of other quality dimensions such as coverage, novelty, or diversity. This balance is critical, since it is possible to achieve a very high precision recommending only one item to a unique user, which would not be a very useful recommender. Because of this, on the one hand, we explore some techniques to combine precision and coverage metrics, an open problem in the area. On the other hand, a family of metrics (correctness) based on the assumption that it is better to avoid a recommendation rather than providing a bad recommendation is proposed herein. In summary, the contributions of this paper are twofold: a taxonomy of techniques that can be applied to some families of recommender systems allowing to include mechanisms to decide if a recommendation should be generated, and a first exploration to the combination of evaluation metrics, mostly focused on measures for precision and coverage. Empiric results show that large precision improvements are obtained when using these approaches at the expense of user and item coverage and with varying levels of novelty and diversityThis work was funded by the national Spanish Government under project TIN2016-80630-P. The authors also acknowledge the very helpful feedback from the three anonymous reviewer

Aspect-based active learning for user preference elicitation in recommender systems

Recommender systems require interactions from users to infer personal preferences about new items. Active learning techniques aim to identify those items that allow eliciting a target user’s preferences more efficiently. Most of the existing techniques base their decisions on properties of the items themselves, for example according to their popularity or in terms of their influence on reducing information variance or entropy within the system. Differently to previous work, in this paper we explore a novel active learning approach focused on opinions about item aspects extracted from user reviews. We thus incorporate textual information so as to decide which items should be considered next in the user preference elicitation process. Experiments on a real-world dataset provide positive results with respect to competitive state of the art methodsThis work was supported by the Spanish Ministry of Science and Innovation (PID2019-108965GB-I00

Explaining recommender systems fairness and accuracy through the lens of data characteristics

The impact of data characteristics on the performance of classical recommender systems has been recently investigated and produced fruitful results about the relationship they have with recommendation accuracy. This work provides a systematic study on the impact of broadly chosen data characteristics (DCs) of recommender systems. This is applied to the accuracy and fairness of several variations of CF recommendation models. We focus on a suite of DCs that capture properties about the structure of the user–item interaction matrix, the rating frequency, item properties, or the distribution of rating values. Experimental validation of the proposed system involved large-scale experiments by performing 23,400 recommendation simulations on three real-world datasets in the movie (ML-100K and ML-1M) and book domains (BookCrossing). The validation results show that the investigated DCs in some cases can have up to 90% of explanatory power – on several variations of classical CF algorithms –, while they can explain – in the best case – about 40% of fairness results (measured according to user gender and age sensitive attributes). Therefore, this work evidences that it is more difficult to explain variations in performance when dealing with fairness dimension than accuracyThis work was supported in part by the Ministerio de Ciencia, Innovación y Universidades (reference: PID2019-108965GB-I00), and in part by H2020 PASSEPARTOUT No. 101016956, Servizi Locali S.P.A. Servizi Locali 2.0, and Ministry of Education, University and Research (references: PON ARS01_00876 Bio-D, PON ARS01_00821 FLET4.0, PON ARS01_00917 OK-INSAID

A unifying and general account of fairness measurement in recommender systems

Fairness is fundamental to all information access systems, including recommender systems. However, the landscape of fairness definition and measurement is quite scattered with many competing definitions that are partial and often incompatible. There is much work focusing on specific – and different – notions of fairness and there exist dozens of metrics of fairness in the literature, many of them redundant and most of them incompatible. In contrast, to our knowledge, there is no formal framework that covers all possible variants of fairness and allows developers to choose the most appropriate variant depending on the particular scenario. In this paper, we aim to define a general, flexible, and parameterizable framework that covers a whole range of fairness evaluation possibilities. Instead of modeling the metrics based on an abstract definition of fairness, the distinctive feature of this study compared to the current state of the art is that we start from the metrics applied in the literature to obtain a unified model by generalization. The framework is grounded on a general work hypothesis: interpreting the space of users and items as a probabilistic sample space, two fundamental measures in information theory (Kullback–Leibler Divergence and Mutual Information) can capture the majority of possible scenarios for measuring fairness on recommender system outputs. In addition, earlier research on fairness in recommender systems could be viewed as single-sided, trying to optimize some form of equity across either user groups or provider/procurer groups, without considering the user/item space in conjunction, thereby overlooking/disregarding the interplay between user and item groups. Instead, our framework includes the notion of statistical independence between user and item groups. We finally validate our approach experimentally on both synthetic and real data according to a wide range of state-of-the-art recommendation algorithms and real-world data sets, showing that with our framework we can measure fairness in a general, uniform, and meaningful wayThis work was supported in part by the Ministerio de Ciencia, Innovación y Universidades, Spain (references PID2019-108965GBI00 and PID2021-124361OB-C32