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What Is Reinforcement Learning from Human Feedback (RLHF)? Definition and How It Works

RLHF uses human preferences to learn a reward signal, then optimizes an AI system against it. Here is the classic pipeline, what it isn't, and its limits.
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Reinforcement learning from human feedback (RLHF) is a family of training methods in which human judgments or preferences are used to shape a learned reward signal. That signal then improves an AI system through reinforcement learning. People do not write a numeric score for every output. They show what they prefer, and a model learns to turn those preferences into a reward.

The classic language-model pipeline

The best-known example is OpenAI’s InstructGPT work (January 27, 2022 announcement, “Aligning language models to follow instructions”, and the accompanying paper “Training language models to follow instructions with human feedback”). It followed three stages. This is a representative pipeline, not a requirement that every RLHF method use the same data format or algorithm.

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  1. Demonstrations and supervised fine-tuning. Labelers write examples of the desired behavior. The model is fine-tuned on them, producing a supervised policy.
  2. Preference comparisons and reward modeling. Labelers compare several outputs for the same prompt. A reward model learns to predict which output they would prefer.
  3. Reinforcement-learning optimization. The policy is optimized to raise the reward that model predicts. InstructGPT used proximal policy optimization (PPO).

The key idea is the middle step. The human signal in InstructGPT was a ranking, and a learned model converted it into the reward used during optimization.

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Why use human feedback at all?

OpenAI’s stated reasoning: “This technique uses human preferences as a reward signal to fine-tune our models, which is important as the safety and alignment problems we are aiming to solve are complex and subjective, and aren’t fully captured by simple automatic metrics.” RLHF is a way to express a training objective through judgments when no simple automatic metric exists.

Anthropic describes the same approach in its April 12, 2022 paper, “Training a Helpful and Harmless Assistant with Reinforcement Learning from Human Feedback”: “We apply preference modeling and reinforcement learning from human feedback (RLHF) to finetune language models to act as helpful and harmless assistants.”

What RLHF does and does not mean

The learned reward is a proxy

The reward model predicts preferences found in its data. A high score does not prove an answer is true, safe, or acceptable to everyone.

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PPO is an example, not the definition

PPO was the optimizer in the InstructGPT setup. Treat it as a method choice in that work, not a necessary part of RLHF.

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It is not limited to chatbots

OpenAI’s earlier “Learning from human preferences” research applied feedback-learned rewards to simulated robotics and Atari tasks. Language-model assistants came later. The two settings differ in domain (text versus simulated control), feedback format (demonstrations versus comparisons), and optimization method.

Numbers often quoted, with context

These figures come from the 2022 InstructGPT paper. They describe its experiments, evaluation prompts, and models. They are historical findings, not guarantees about current systems.

Figure What it means
85 ± 3% How often 175B InstructGPT outputs were preferred to 175B GPT-3 outputs on the study’s test set.
21% vs. 41% Closed-domain hallucination rate (information not in the input): InstructGPT versus GPT-3, on the paper’s tasks.
About 25% fewer Approximate reduction in toxic outputs versus GPT-3 when prompted to be respectful, under the paper’s evaluation.
40 contractors Size of the labeling team for the study.
About 900 bits Evaluator feedback used in OpenAI’s human-preferences demonstration of a simulated backflip, which also took under an hour of evaluator time and about 70 hours of simulated experience. This is a detail of that demo, not a general data requirement.
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Limitations

  • Whose preferences? OpenAI notes that the data reflected its labelers, researchers, and policies, and that “these different sources of influence on the data do not guarantee our models are aligned to the preferences of any broader group.”
  • Remaining flaws. The InstructGPT models could still produce toxic or biased outputs and make up facts. Training was in English and culturally limited. The paper presents progress, not complete alignment, and documents tradeoffs across evaluation tasks.
  • Evaluators can be fooled. In OpenAI’s robotics work, a simulated agent seemed to grasp an object by placing its manipulator between the camera and the object. Optimizing against an imperfect evaluator can reward appearance instead of the intended behavior.

When you cite RLHF results, name the model, dataset or task, comparator, and date.

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