Concept

Training Objective of Masked Language Modeling (MLM)

Given an original text sequence x\mathbf{x} and its corrupted version xˉ\bar{\mathbf{x}}, optimizing a model to predict x\mathbf{x} based on xˉ\bar{\mathbf{x}} can be thought of as an autoencoding-like process. The fundamental training objective is to maximize the reconstruction probability Pr(xxˉ)\Pr(\mathbf{x}|\bar{\mathbf{x}}). However, because there is a simple position-wise alignment between the two sequences, an unmasked token in xˉ\bar{\mathbf{x}} is the exact same as the token in x\mathbf{x} at the same position. Since there is no need to consider the prediction for these unmasked tokens, the training objective is simplified to only maximize the probabilities for the masked tokens.

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Updated 2026-07-02

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Ch.1 Pre-training - Foundations of Large Language Models

Foundations of Large Language Models

Foundations of Large Language Models Course

Computing Sciences

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