João Vinagre

Abstract
The typical offline protocol to evaluate recommendation algorithms is to collect a dataset of user-item interactions and then use a part of this dataset to train a model, and the remaining data to measure how closely the model recommendations match the observed user interactions. This protocol is straightforward, useful and practical, but it only provides snapshot performance. We know, however, that online systems evolve over time. In general, it is a good idea that models are frequently retrained with recent data. But if this is the case, to what extent can we trust previous evaluations? How will a model perform when a different pattern (re)emerges? In this paper we propose a methodology to study how recommendation models behave when they are retrained. The idea is to profile algorithms according to their ability to, on the one hand, retain past patterns - stability - and, on the other hand, (quickly) adapt to changes - plasticity. We devise an offline evaluation protocol that provides detail on the long-term behavior of models, and that is agnostic to datasets, algorithms and metrics. To illustrate the potential of this framework, we present preliminary results of three different types of algorithms on the GoodReads dataset that suggest different stability and plasticity profiles depending on the algorithmic technique, and a possible trade-off between stability and plasticity. We further discuss the potential and limitations of the proposal and advance some possible improvements. © 2026 Copyright held by the owner/author(s).

Abstract

Abstract
The European Union (EU) Digital Services Act (DSA) has introduced a novel set of rules for online platforms and search engines, with significant implications for the Recommender Systems community. Through its data access mechanisms, the DSA invites researchers to request both publicly available and private data from Very Large Online Platforms (VLOPs) and Very Large Search Engines (VLOSEs) - those with more than 45 million active recipients in the EU - to investigate systemic risks associated with the dissemination of illegal content, risks to the exercise of fundamental rights, and negative effects on electoral processes, public health, and gender-based violence. This tutorial is aimed at researchers who are interested in submitting such data access requests and will provide them with the knowledge to do so by introducing the relevant definitions and provisions of the DSA, and addressing the most important procedural steps to obtain data access and will provide attendees with a comprehensive understanding of the DSA's data access implications for RecSys research. The tutorial targets researchers, practitioners, and students in understanding current developments in online platform regulation in Europe and their impact on RecSys research.

Abstract
The Digital Services Act (DSA) establishes a regulatory framework for online platforms and search engines in the European Union, focusing on mitigating systemic risks such as illegal content dissemination, fundamental rights violations, and impacts on electoral processes, public health, and gender-based violence. Very Large Online Platforms (VLOPs) and Very Large Search Engines (VLOSEs), defined as those with over 45 million active recipients, must provide data access for research to enable investigations into these risks and the development of solutions. This tutorial is tailored for the UMAP community, addressing the implications of the DSA for user modelling research. It will cover the DSA's key provisions and definitions, outline the procedural steps for accessing VLOP and VLOSE data, and discuss the technical aspects of data access requests. Participants will also explore the challenges and opportunities involved in working with this data. By the end of the tutorial, attendees will have a thorough understanding of the DSA's data access provisions, the technical and procedural requirements for accessing VLOP and VLOSE data, and the regulation's implications for user modelling research. They will be equipped to navigate the complexities of the DSA and contribute to the development of responsible and transparent online platforms.Further information and resources about the tutorial are available on the website: https://erasmopurif.com/tutorial-dsa-umap25/.

Abstract