Tutorials and talks (current)

• Fairness, Accountability, Transparency and Ethics of Algorithmic Hiring. Masterclass prepared with the FINDHR Project, 2025.
• Human factors and algorithmic fairness. Keynote at 2024 European Conference on Information Retrieval (ECIR).
• Disparate effects of recommender systems. Invited talk at 2022 ACM-W Greece Winter School based on PhD work of David Solans and Francesco Fabbri.

⚖️ Risk Assessment of Criminal Recidivism (2021-present)

Work led by PhD student Marzieh Karimi-Haghighi with Songül Tolan, Marius Miron, Emilia Gomez, Antonio Andres-Pueyo, Kristian Lum, and Manuel Portela, and MSc student Ashwin Singh.

• Ashwin Singh, Carlos Castillo: Model Multiplicity and Predictive Arbitrariness in Recidivism Risk Assessment. Pre-print [arxiv].
• Manuel Portela, Carlos Castillo, Songül Tolan, Marzieh Karimi-Haghighi, Antonio Andres Pueyo. A Comparative User Study of Human Predictions in Algorithm-Supported Recidivism Risk Assessment. Artificial Intelligence and Law. Springer, 2024. [doi|springer|preprint]
• Marzieh Karimi-Haghighi, Carlos Castillo, Songül Tolan, Kristian Lum: Effect of Conditional Release on Violent and General Recidivism: A Causal Inference Study. Journal of Experimental Criminology. Springer, 2023. [doi|springer]
• Marzieh Karimi-Haghighi, Carlos Castillo: Enhancing a recidivism prediction tool with machine learning: effectiveness and algorithmic fairness. ICAIL 2021 (Short papers), pp. 210-214, 2021. [acm].
• Marzieh Karimi-Haghighi, Carlos Castillo: Efficiency and Fairness in Recurring Data-Driven Risk Assessments of Violent Recidivism. In Proc. of SIGAPP Symposium on Applied Computing (SAC), pp. 994-1002, ACM Press, 2021. [doi|acm]
• Marius Miron, Songül Tolan, Emilia Gomez, Carlos Castillo: Evaluating causes of algorithmic bias in juvenile criminal recidivism. Journal on Artificial Intelligence and Law, Springer, 2020. [doi]
• Marius Miron, Songül Tolan, Emilia Gomez, Carlos Castillo: Why Machine Learning May Lead to Unfairness: Evidence from Risk Assessment for Juvenile Justice in Catalonia. In Proc. of International Conference on Artificial Intelligence and Law (ICAIL), Montréal, Canada, pp. 83-92. ACM Press, 2019. [slides|acm] Best paper award.

🩺 Fairness in Digital Health (2020-present)

• Ioannis Bilionis, Ricardo Berrios, Antonio de Arriba Muñoz, Luis Fernandez-Luque, Carlos Castillo. A Framework for Assessing Algorithmic Discrimination Risks in Training Data: A Case of Pediatric Type 1 Diabetes. To appear in JAMIA Open.
• Ioannis Bilionis, Luis Fernandez-Luque, Carlos Castillo, Ricardo C. Berrios. An empirical evaluation of the risks of AI model updates using clinical data: stability, arbitrariness, and fairness. To appear in IEEE EMBC 2026. [arxiv]
• Ioannis Bilionis, Ricardo C. Berrios, Luis Fernandez-Luque, Carlos Castillo. Disparate Model Performance and Stability in Machine Learning Clinical Support for Diabetes and Heart Diseases. In Proc. AMIA 2025. [arxiv]
• Ioannis Bilionis, Luis Fernandez-Luque and Carlos Castillo: A Survey on Public Data Sets Related to Chronic Diseases. In Proc. CBMS, 2023.
• Teodora Sandra Buda, Joao Guerreiro, Jesus Omana Iglesias, Carlos Castillo, Oliver Smith, Aleksandar Matic: Foundations for Fairness in Digital Health Apps. Frontiers in Digital Health, August 2022. [doi|alt].
• Gemma Galdon Clavell, Mariano Martín Zamorano, Carlos Castillo, Oliver Smith and Aleksandar Matic: Auditing Algorithms: On Lessons Learned and the Risks of Data Minimization. In Proc. of the AIES 2020 conference. ACM Press. [acm]

🧠 Decision Support (2022-present)

• Marina Estévez-Almenzar, Ricardo Baeza-Yates, Carlos Castillo. A Comparison of Human and Machine Learning Errors in Face Recognition. In TRUST-AI, The European Workshop on Trustworthy AI. [arxiv].
• Marina Estévez-Almenzar, Ricardo Baeza-Yates, Carlos Castillo. Human Response to Decision Support in Face Matching: The Influence of Task Difficulty and Machine Accuracy. To appear in HHAI 2025.
• David Solans, Andrea Beretta, Manuel Portela, Carlos Castillo, Anna Monreale. Human Response to an AI-Based Decision Support System: A User Study on the Effects of Accuracy and Bias. Technical Report, UPF, 2022. [arxiv]

🖥️ Other Topics of Algorithmic Bias (2020-present)

• Dinesh Ayyappan, Carlos Castillo: Discriminatory Compliance: How LLMs Answer Queries from Protected Groups. Pre-print [arxiv].
• Arpit Merchant, Carlos Castillo: Disparity, Inequality, and Accuracy Tradeoffs in Graph Neural Networks for Node Classification. In Proc. CIKM 2023. ACM Press. [doi|arxiv]
• Alexandra Olteanu, Michael Ekstrand, Carlos Castillo, Jina Suh. Responsible AI Research Needs Impact Statements Too. Pre-print [arxiv|blogpost]
• David Solans, Francesco Fabbri, Caterina Calsamiglia, Carlos Castillo, Francesco Bonchi: Comparing equity and effectiveness of different algorithms in an application for the room rental market. In Proc. of AIES 2021, pp. 978-988. AAAI/ACM. [video|doi]

Past topics

🎓 University Admissions and Evaluation (2020-present)

Work led by PhD students Francielle Marques and Marzieh Karimi-Haghighi, with Giorgio Barnabo, Corinna Hertweck, Michalis Mathioudakis, Davinia Hernandez-Leo, and Sergio Celis.

• Francielle Marques, Davinia Hernandez-Leo, Carlos Castillo. Beyond bias in student satisfaction surveys: exploring the role of grades and satisfaction with the learning design. Journal of New Approaches in Educational Research 14(9), 2025. [doi]
• Francielle Marques, Davinia Hernandez-Leo, Carlos Castillo. Measuring gender bias in student satisfaction in higher education: A cross-department study. Cogent Education 11, Taylor and Francis, 2024. [doi]
• Marzieh Karimi-Haghighi, Carlos Castillo, Davinia Hernández-Leo: A Causal Inference Study on the Effects of First Year Workload on the Dropout Rate of Undergraduates. In AIED 2022. Springer. [springer]. Nominated for Best Paper Award
• Corinna Hertweck, Carlos Castillo, Michael Mathioudakis. Designing Affirmative Action Policies under Uncertainty. Journal of Learning Analytics 9(2), Society for Learning Analytics Research, 2022. [doi|jla]. Based on the MSc Thesis of Corinna Hertweck at Helsinki University.
• Marzieh Karimi-Haghighi, Carlos Castillo, Davinia Hernandez-Leo, Veronica Moreno Oliver: Predicting Early Dropout: Calibration and Algorithmic Fairness Considerations. ADORE Workshop at LAK’21 [arxiv]
• Michael Mathioudakis, Carlos Castillo, Giorgio Barnabo, Sergio Celis: Affirmative Action Policies for Top-k Candidates Selection, With an Application to the Design of Policies for University Admissions. To appear in the ACM Symposium on Applied Computing (SAC), Brno, Czech Republic, March 2020. [arxiv]
• Corinna Hertweck, Carlos Castillo, Michael Mathioudakis: Towards Data-Driven Affirmative Action Policies under Uncertainty. In Fairness, Accountability, and Transparency in Educational Data Cyberspace (FATED) Workshop. [arxiv]

🎵 Diversity in Music Recommendation (2020-2024)

Work led by PhD student Lorenzo Porcaro, with Dougal Shakespeare and Emilia Gomez.

• Lorenzo Porcaro, Carlos Castillo, Emilia Gomez, João Vinagre: Fairness and Diversity in Information Access Systems. To be presented at the European Workshop on Algorithmic Fairness (EWAF). [request by mail]
• Lorenzo Porcaro, Emilia Gómez, Carlos Castillo. Assessing the Impact of Music Recommendation Diversity on Listeners: A Longitudinal Study. ACM Transactions on Recommender Systems, 2024. [doi|acm|arxiv]
• Lorenzo Porcaro, Emilia Gómez, Carlos Castillo: Perceptions of Diversity in Electronic Music: the Impact of Listener, Artist, and Track Characteristics. In Proc. of CSCW 2022, Vol. 6, Issue CSCW1, Article 109, pp. 1-26. ACM Press. [doi|acm|arxiv]
  • Lorenzo Porcaro, Emilia Gómez, Carlos Castillo. Diversity in the Music Listening Experience: Insights from Focus Group Interviews. Proc. of ACM SIGIR Conference on Human Information Interaction and Retrieval (CHIIR), pp 272-276, ACM Press. [doi|arxiv]
• Dougal Shakespeare, Lorenzo Porcaro, Emilia Gómez, Carlos Castillo: Exploring Artist Gender Bias in Music Recommendation. 2nd Workshop on the Impact of Recommender Systems., 2020.

🔗 Link-Based Recommender Systems (2021-2022)

Work led by PhD student Francesco Fabbri with Yanhao Wang, Marialuisa Croci, Francesco Bonchi, Ludovico Boratto, Michalis Mathioudakis.

• Francesco Fabbri, Yanhao Wang, Francesco Bonchi, Carlos Castillo and Michael Mathioudakis. Rewiring What-to-Watch-Next Recommendations to Reduce Radicalization Pathways. In Proc. TheWebConf 2022. ACM Press. [doi|arxiv|press] Best paper award.
• Francesco Fabbri, Marialuisa Croci, Francesco Bonchi, Carlos Castillo. Exposure Inequality in People Recommender Systems: The Long-Term Effects. To appear in ICWSM 2022. AAAI Press. [arxiv]
• Francesco Fabbri, Francesco Bonchi, Ludovico Boratto, Carlos Castillo. The Effect of Homophily on Disparate Visibility of Minorities in People Recommender Systems. ICWSM, AAAI [request by mail]

🔎 Fair Ranking (2016-2021)

Work led by PhD student Meike Zehlike with Tom Sühr, Sara Hajian, Ricardo Baeza-Yates, Francesco Bonchi, and Mohamed Megahed

• Fairness and Transparency in Ranking. Keynote at Data and Bias workshop at CIKM. In SIGIR Forum, Vol. 52. No. 2, December 2018, pages 64-71. [updated slides (2021)|sigir forum]
• Meike Zehlike, Carlos Castillo: Reducing Disparate Exposure in Ranking: A Learning To Rank Approach. In WWW Short papers, Taipei, Taiwan. [arxiv]
• Meike Zehlike, Francesco Bonchi, Carlos Castillo, Sara Hajian, Mohamed Megahed and Ricardo Baeza-Yates: FA*IR: A Fair Top-k Ranking Algorithm. In Proc. of CIKM. Singapore, 2017. ACM Press. [acm|arxiv|Python Library|Java Library|ElasticSearch Plug-in]
  • Meike Zehlike, Tom Sühr, Carlos Castillo: A Note on the Significance Adjustment for FA*IR with Two Protected Groups. arXiv, 2020. (Corrects the significance adjustment test.)

Fair search tools

• Meike Zehlike, Tom Sühr, Carlos Castillo, Ivan Kitanovski: FairSearch: A Tool For Fairness in Ranked Search Results (Pre-print / Demo paper) [arxiv]
• FA*IR (Post-Processing for Fair Ranking):
  • FA*IR library in Python
  • FA*IR library in Java
  • FA*IR plug-in for Elasticsearch

• DELTR (Disparate Exposure in Learning To Rank):
  • DELTR library in Python
  • DELTR library in Java
  • DELTR adapter for Elasticsearch
• https://fairnessmeasures.github.io/

See also: Machnamh – A Machine Learning Toolkit for auditing fairness and facilitating a reflective analysis of ML models.

👁️‍🗨️ Social data biases (2018-2019)

Work led by Alexandra Olteanu with Fernando Diaz and Emre Kıcıman, published in 2018-2019.

• Alexandra Olteanu, Carlos Castillo, Fernando Diaz, Emre Kıcıman: “Social Data: Biases, Methodological Pitfalls, and Ethical Boundaries” Frontiers in Big Data, Volume 2, July 2019 [ssrn/previous version].
  • Alexandra Olteanu, Emre Kıcıman, Carlos Castillo, Fernando Diaz: A Critical Review of Online Social Data: Biases, Methodological Pitfalls, and Ethical Boundaries. Tutorial at WSDM 2018, WWW 2018, SDM 2018.
• Carlos Castillo: “A Brief Overview of Sources and Manifestations of Bias When Working with Social Data.” Summary of the tutorial coordinated by Alexandra Olteanu, for the Russian Summer School on Information Retrieval (RuSSIR). August 2018.

🖥️ Algorithmic bias (2016-2018)

Work led by Sara Hajian with Francesco Bonchi, published in 2016.

• Sara Hajian, Francesco Bonchi, Carlos Castillo: Algorithmic bias: from discrimination discovery to fairness-aware data mining. Tutorial at KDD 2016. [acm|7-pages outline].
  • Slides: Parts I and II: discrimination discovery, Parts III and IV: fairness-aware data mining.
  • Video: Part I: Introduction and Context, Part II: Discrimination Discovery, Part III: Fairness-Aware Data Mining and Part IV: Challenges and Directions for Future Research.
• Carlos Castillo: Algorithmic Discrimination Talk at MPI Saarbrücken, Germany, April 2019
• Carlos Castillo: “Algorithmic Discrimination.” Talk at BCN Analytics Data and Ethics event, April 2018. [VIDEO]

🗯️ Exclusion and Hate Speech (2013-2018)

• Valerio Lorini, Javier Rando, Diego Saez-Trumper, Carlos Castillo: Uneven Coverage of Natural Disasters in Wikipedia: the Case of Floods. ISCRAM, Virginia, USA. [press release|iscram|arxiv].
• Ugur Kursuncu, Manas Gaur, Carlos Castillo, Amanuel Alambo, Krishnaprasad Thirunarayan, Valerie Shalin, Dilshod Achilov, I. Budak Arpinar, Amit Sheth. Modeling Islamist Extremist Communications on Social Media using Contextual Dimensions: Religion, Ideology, and Hate. In Proc. of CSCW 2019, Austin, Texas. [arxiv|doi|slides]
• Alexandra Olteanu, Carlos Castillo, Jeremy Boy and Kush Varshney: The Effect of Extremist Violence on Hateful Speech Online. In ICWSM, Stanford, CA, June 2018 [aaai|arxiv|slides|data and code|forbes].
• Diego Sáez-Trumper, Carlos Castillo and Mounia Lalmas: Social Media News Communities: Gatekeeping, Coverage, and Statement Bias (+ supplementary material). In CIKM 2013 (short paper) [acm|slides|mirror|bib]