Golnoosh Farnadi

marylou.fauchard@mila.quebec

Marylou Fauchard

Master's Research - Université de Montréal

Principal supervisor :

Margarida Carvalho

Prakhar Ganesh

PhD - McGill University

prakhar.ganesh@mila.quebec

Research Intern - McGill University

mira.kandlikar-bloch@mail.mcgill.ca

saber.malekmohammadi@mila.quebec

Aly Kassem

Collaborating researcher - UWindsor

PhD - McGill University

Co-supervisor :

Collaborating researcher - McGill University

Yash Raj More

Collaborating Alumni - Université de Montréal

Selin Omur

Collaborating researcher - McGill University

siomur@gmail.com

Sophia Osborne

Research Intern - McGill University

Edie Pearman

Independent visiting researcher - McGill University university

Nahian Rahman

Research Intern - McGill University

Soumya Sharma

PhD - McGill University

Co-supervisor :

Adriana Romero Soriano

Website

Google Scholar

Zhuan Shi

Postdoctorate - McGill University

William St-Arnaud

PhD - Université de Montréal

Co-supervisor :

Margarida Carvalho

Afaf Taïk

Collaborating Alumni - Université de Sherbrooke

Jesse Thibodeau

Independent visiting researcher - HEC Montréal

Emma Tomiuk Tomiuk

Master's Research - McGill University

Publications

Adaptation, Comparison and Practical Implementation of Fairness Schemes in Kidney Exchange Programs

William St-Arnaud

Margarida Carvalho

In Kidney Exchange Programs (KEPs), each participating patient is registered together with an incompatible donor. Donors without an incompat… (see more)ible patient can also register. Then, KEPs typically maximize overall patient benefit through donor exchanges. This aggregation of benefits calls into question potential individual patient disparities in terms of access to transplantation in KEPs. Considering solely this utilitarian objective may become an issue in the case where multiple exchange plans are optimal or near-optimal. In fact, current KEP policies are all-or-nothing, meaning that only one exchange plan is determined. Each patient is either selected or not as part of that unique solution. In this work, we seek instead to find a policy that contemplates the probability of patients of being in a solution. To guide the determination of our policy, we adapt popular fairness schemes to KEPs to balance the usual approach of maximizing the utilitarian objective. Different combinations of fairness and utilitarian objectives are modelled as conic programs with an exponential number of variables. We propose a column generation approach to solve them effectively in practice. Finally, we make an extensive comparison of the different schemes in terms of the balance of utility and fairness score, and validate the scalability of our methodology for benchmark instances from the literature.

2025-08-01

European Journal of Operational Research (published)

doi.org

arxiv.org

Hallucination Detox: Sensitivity Dropout (SenD) for Large Language Model Training

Shahrad Mohammadzadeh

Juan David Guerra

Marco Bonizzato

Reihaneh Rabbany

2025-07-01

Proceedings of the 63rd Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers) (published)

doi.org

Low-Rank Adaptation Secretly Imitates Differentially Private SGD

Saber Malekmohammadi

As pre-trained language models grow in size, full fine-tuning their parameters on task adaptation data becomes increasingly impractical. To … (see more)address this challenge, some methods for low-rank adaptation of language models have been proposed, e.g. LoRA, which incorporates trainable low-rank decomposition matrices into only some parameters of the pre-trained model, called adapters. This approach significantly reduces the number of trainable parameters compared to fine-tuning all parameters or adapters. In this work, we look at low-rank adaptation method from the lens of data privacy. We show theoretically that the low-rank adaptation used in LoRA is equivalent to fine-tuning adapters with noisy batch gradients - just like what DPSGD algorithm does. We also quantify the variance of the injected noise as a decreasing function of adaptation rank. By establishing a Berry-Esseen type bound on the total variation distance between the injected noise distribution and a Gaussian noise distribution with the same variance, we show that the dynamics of low-rank adaptation is very close to when DPSGD is performed w.r.t the adapters. Following our theoretical findings and approved by our experimental results, we show that low-rank adaptation provides robustness to membership inference attacks w.r.t the fine-tuning data.

2025-06-10

ICML.cc/2025/Workshop/MemFM (published)

Algorithmic Fairness Through the Lens of Metrics and Evaluation (AFME) 2024

Miriam Rateike

Awa Dieng

Jamelle Watson-Daniels

Ferdinando Fioretto

2025-04-23

Proceedings of the Algorithmic Fairness Through the Lens of Metrics and Evaluation (published)

proceedings.mlr.press

Different Horses for Different Courses: Comparing Bias Mitigation Algorithms in ML

Prakhar Ganeesh

Usman Gohar

Lu Cheng

With fairness concerns gaining significant attention in Machine Learning (ML), several bias mitigation techniques have been proposed, often … (see more)compared against each other to find the best method. These benchmarking efforts tend to use a common setup for evaluation under the assumption that providing a uniform environment ensures a fair comparison. However, bias mitigation techniques are sensitive to hyperparameter choices, random seeds, feature selection, etc., meaning that comparison on just one setting can unfairly favour certain algorithms. In this work, we show significant variance in fairness achieved by several algorithms and the influence of the learning pipeline on fairness scores. We highlight that most bias mitigation techniques can achieve comparable performance, given the freedom to perform hyperparameter optimization, suggesting that the choice of the evaluation parameters-rather than the mitigation technique itself-can sometimes create the perceived superiority of one method over another. We hope our work encourages future research on how various choices in the lifecycle of developing an algorithm impact fairness, and trends that guide the selection of appropriate algorithms.

2025-04-23

Proceedings of the Algorithmic Fairness Through the Lens of Metrics and Evaluation (published)

doi.org

arxiv.org

Multilingual Hallucination Gaps

Cléa Chataigner

Afaf Taïk

2025-04-23

Proceedings of the Algorithmic Fairness Through the Lens of Metrics and Evaluation (published)

proceedings.mlr.press

Privacy-Preserving Group Fairness in Cross-Device Federated Learning

Sikha Pentyala

Nicola Neophytou

Anderson Nascimento

Martine De Cock

Group fairness ensures that the outcome of machine learning (ML) based decision making systems are notbiased towards a certain group of peop… (see more)le defined by a sensitive attribute such as gender or ethnicity. Achievinggroup fairness in Federated Learning (FL) is challenging because mitigating bias inherently requires usingthe sensitive attribute values of all clients, while FL is aimed precisely at protecting privacy by not givingaccess to the clients’ data. As we show in this paper, this conflict between fairness and privacy in FL can beresolved by combining FL with Secure Multiparty Computation (MPC) and Differential Privacy (DP). Tothis end, we propose a privacy-preserving approach to calculate group fairness notions in the cross-device FLsetting. Then, we propose two bias mitigation pre-processing and post-processing techniques in cross-deviceFL under formal privacy guarantees, without requiring the clients to disclose their sensitive attribute values.Empirical evaluations on real world datasets demonstrate the effectiveness of our solution to train fair andaccurate ML models in federated cross-device setups with privacy guarantees to the users.

2025-04-23

Proceedings of the Algorithmic Fairness Through the Lens of Metrics and Evaluation (published)

proceedings.mlr.press

Rethinking Hallucinations: Correctness, Consistency, and Prompt Multiplicity

Prakhar Ganesh

Reza Shokri

Large language models (LLMs) are known to "hallucinate" by generating false or misleading outputs. Hallucinations pose various harms, from e… (see more)rosion of trust to widespread misinformation. Existing hallucination evaluation, however, focuses only on "correctness" and often overlooks "consistency", necessary to distinguish and address these harms. To bridge this gap, we introduce _prompt multiplicity_, a framework for quantifying consistency through prompt sensitivity. Our analysis reveals significant multiplicity (over 50% inconsistency in benchmarks like Med-HALT), suggesting that hallucination-related harms have been severely underestimated. Furthermore, we study the role of consistency in hallucination detection and mitigation. We find that: (a) detection techniques capture consistency, not correctness, and (b) mitigation techniques like RAG can introduce additional inconsistencies. By integrating prompt multiplicity into hallucination evaluation, we provide an improved framework of potential harms and uncover critical limitations in current detection and mitigation strategies.

2025-03-05

ICLR.cc/2025/Workshop/BuildingTrust (accepted)

On the Role of Prompt Multiplicity in LLM Hallucination Evaluation

Prakhar Ganesh

Reza Shokri

Large language models (LLMs) are known to "hallucinate" by generating false or misleading outputs. Existing hallucination benchmarks often o… (see more)verlook prompt sensitivity, due to stable accuracy scores despite prompt variations. However, such stability can be misleading. In this work, we introduce prompt multiplicity--the multiplicity of individual hallucinations depending on the input prompt--and study its role in LLM hallucination benchmarks. We find severe multiplicity, with even more than 50% of responses changing between correct and incorrect answers simply based on the prompt for certain benchmarks, like Med-HALT. Prompt multiplicity also gives us the lens to distinguish between randomness in generation and consistent factual inaccuracies, providing a more nuanced understanding of LLM hallucinations and their real-world harms. By situating our discussion within existing hallucination taxonomies--supporting their quantification--and exploring its relationship with uncertainty in generation, we highlight how prompt multiplicity fills a critical gap in the literature on LLM hallucinations.

2025-03-05

ICLR.cc/2025/Workshop/BuildingTrust (accepted)

Alireza Dehghanpour Farashah

UNLEARNING GEO-CULTURAL STEREOTYPES IN MULTILINGUAL LLMS

Aditi Khandelwal

Negar Rostamzadeh

As multilingual generative models become more widely used, most safety and fairness evaluation techniques still focus on English-language re… (see more)sources, while overlooking important cross-cultural factors. This limitation raises concerns about fairness and safety, particularly regarding geoculturally situated stereotypes that hinder the models’ global inclusivity. In this work, we present preliminary findings on the impact of stereotype unlearning across languages, specifically in English, French, and Hindi. Using an adapted version of the SeeGULL dataset, we analyze how unlearning stereotypes in one language influences other languages within multilingual large language models. Our study evaluates two model families, Llama-3.1-8B and Aya-Expanse-8B, to assess whether unlearning in one linguistic context transfers across languages, potentially mitigating or exacerbating biases in multilingual settings.

2025-03-05

ICLR.cc/2025/Workshop/BuildingTrust (accepted)

Bridging Causality, Individual Fairness, and Adversarial Robustness in the Absence of Structural Causal Model

Ahmad Reza Ehyaei

Samira Samadi

Despite the essential need for comprehensive considerations in responsible AI, factors such as robustness, fairness, and causality are often… (see more) studied in isolation. Adversarial perturbation, used to identify vulnerabilities in models, and individual fairness, aiming for equitable treatment of similar individuals, despite initial differences, both depend on metrics to generate comparable input data instances. Previous attempts to define such joint metrics often lack general assumptions about data and were unable to reflect counterfactual proximity. To address this, our paper introduces a \emph{causal fair metric} formulated based on causal structures encompassing sensitive attributes and protected causal perturbation. To enhance the practicality of our metric, we propose metric learning as a method for metric estimation and deployment in real-world problems in the absence of structural causal models. We also demonstrate the applications of the causal fair metric in classifiers. Empirical evaluation of real-world and synthetic datasets illustrates the effectiveness of our proposed metric in achieving an accurate classifier with fairness, resilience to adversarial perturbations, and a nuanced understanding of causal relationships.

2025-02-03

TMLR (accepted)

What Secrets Do Your Manifolds Hold? Understanding the Local Geometry of Generative Models

Ahmed Imtiaz Humayun

Ibtihel Amara

Candice Schumann

Cristina Nader Vasconcelos

Deepak Ramachandran

Negar Rostamzadeh

Junfeng He

Mohammad Havaei

Katherine Heller

2025-01-22

ICLR.cc/2025/Conference (poster)