On the Impossibility of Retrain Equivalence in Machine Unlearning

On the Impossibility of Retrain Equivalence in Machine Unlearning

🎙 Robust and Interpretable Machine Learning Lab 👥 1K 📅 December 8, 2025 ⏱ 68 min 👁 47 📄 literature review 🧭 2026-08-16
Available in: English (current) Français

Keywords

machine unlearningretrain equivalencepath dependencemulti-stage trainingLLM

Summary

This video is a journal club presentation by Arian Komaei, discussing the paper ‘On the Impossibility of Retrain Equivalence in Machine Unlearning’. The central claim is that modern multi-stage training pipelines (e.g., pretraining, instruction tuning, safety tuning) make the ideal of retrain equivalence—where an unlearned model behaves exactly like one retrained without the forgotten data—impossible. The presenters explain that the order of training stages creates path dependence, meaning the model’s behavior after unlearning depends on the sequence of training steps. They illustrate this with experiments on Llama and Qwen models, showing that different training orders lead to significantly different unlearning outcomes, with accuracy drops up to 20%. The theoretical part involves a simplified linear regression setup to demonstrate that gradient-based unlearning methods cannot universally achieve retrain equivalence. The discussion also touches on practical implications, such as the difficulty of unlearning safety training if it was the last stage, and the need to rethink the goals of machine unlearning. The presentation is informal and includes side discussions about related work and personal anecdotes.

172 words

Critical Evaluation

Value of the Information & Strength of the Argument

The video provides a valuable overview of a recent research paper, highlighting a fundamental limitation of current machine unlearning approaches. The argumentation is based on both theoretical analysis and empirical evidence, which strengthens the claims. The presenters effectively convey the core idea of path dependence and its implications. However, the discussion is somewhat unstructured and includes speculative remarks, which slightly weakens the overall rigor. The value lies in raising awareness of the challenges in machine unlearning and prompting a re-evaluation of the field’s objectives.

Scientific Rigor, Source Quality, Title Accuracy

The presentation is based on a single arXiv preprint, which is not peer-reviewed. The sources cited are the paper itself and the presenter’s LinkedIn profile. The title of the video matches the content accurately. The discussion is technical and assumes familiarity with machine learning concepts. The informal style and lack of visual aids (beyond the paper’s figures) may reduce clarity, but the core message is conveyed. The adequacy between title and content is high.

173 words

Title / Content Match

The title accurately reflects the content, which focuses on the theoretical and empirical impossibility of achieving retrain equivalence in machine unlearning.

Quality & Reliability

7/10

The video is a journal club presentation of a recent arXiv paper. The discussion is technical and grounded in the paper's theoretical and empirical results. However, the presentation is informal and includes speculative remarks, and the video quality is low (few views). The paper itself is a preprint and not peer-reviewed, which limits the certainty of the findings.

Key Moments

Cited Sources

Concurring Sources

Dissenting Sources

  • Towards Unbounded Machine Unlearning — This paper proposes a method that claims to achieve approximate unlearning, which contrasts with the impossibility result presented in the video.

Contribution & Novelties

The video presents a novel perspective on machine unlearning by highlighting the fundamental impossibility of retrain equivalence in multi-stage training pipelines. It provides both theoretical and empirical evidence, which is a significant contribution to the field. The discussion also raises important questions about the goals of unlearning and suggests that the field needs to reconsider its objectives.

Pour aller plus loin :

86 words

Radar Profile

The radar profile shows high scores in technical level and information quality, but lower in reliability due to the informal presentation and reliance on a preprint. The overall balance suggests a technically strong but not fully polished presentation.

Reliability 6/10