After the initial Supervised Fine-Tuning (SFT) step, the language model learns to follow instructions or specific formats. However, SFT alone often falls short of capturing the subtleties of human preferences regarding helpfulness, honesty, and harmlessness. Simply mimicking demonstration data doesn't guarantee alignment with these complex, often subjective, values. To bridge this gap, Reinforcement Learning from Human Feedback (RLHF) introduces a mechanism to directly incorporate human judgments into the model's optimization process. The foundation of RLHF is the collection of high-quality human preference data, which serves as the training signal for a subsequent Reward Model (RM). This section details the process of acquiring this critical preference data.

The Goal: Capturing Relative Preferences

The core idea is not to ask humans to write "good" responses (which is closer to SFT data generation) or assign absolute scores (which can be inconsistent across labelers and prompts). Instead, the standard approach involves collecting pairwise comparisons. Given a specific input prompt, the model generates two or more candidate responses. Human labelers are then asked to choose which response they prefer based on predefined criteria (e.g., helpfulness, coherence, safety). This relative judgment tends to be more reliable and easier for humans to provide consistently than absolute scoring.

Generating Prompts and Responses

The process begins with a set of diverse prompts. These prompts should ideally mirror the types of inputs the final aligned model is expected to handle. Sources for prompts can include:

Real user queries logged from previous interactions (if available and privacy permits).
Synthetically generated prompts designed to cover specific domains or behaviors.
Prompts extracted from existing datasets used for evaluation or SFT.
Prompts written specifically by the annotation team to target known model weaknesses or desired capabilities.

It's important to ensure diversity in prompt length, topic, complexity, and interaction style (e.g., question answering, summarization, creative writing, conversational turns).

Once a prompt is selected, the SFT model (the model resulting from the previous fine-tuning stage) is used to generate multiple candidate responses for that prompt. Typically, at least two responses ( $y_1, y_2$ ) are generated for a given prompt $x$ . Techniques like varying the decoding temperature or using top-k/top-p sampling can encourage diversity between the generated responses.

Core loop for generating a single preference data point.

The Labeling Interface and Task

Human labelers are presented with the prompt $x$ and the two generated responses, $y_1$ and $y_2$ . The interface should clearly display the prompt and both responses side-by-side. To mitigate presentation bias, the order of responses (left vs. right, top vs. bottom) should be randomized for each comparison task.

Labelers are given specific instructions and criteria for making their choice. These criteria often revolve around:

Helpfulness: Does the response accurately and completely address the prompt?
Honesty/Accuracy: Is the information provided factual and not misleading?
Harmlessness: Does the response avoid toxic, biased, or inappropriate content?
Coherence/Readability: Is the response well-written and easy to understand?

Labelers select the response ( $y_chosen$ ) they judge to be superior according to these criteria. The other response becomes the rejected response ( $y_{rejected}$ ). Interfaces might also allow labelers to indicate if both responses are equally good/bad, or if neither is acceptable. Some setups might also ask for free-text explanations for the preference, which can be valuable for quality control and understanding failure modes, although this increases annotation cost.

Structuring the Preference Data

The outcome of this process is a dataset of preference tuples. Each tuple typically contains the prompt, the preferred (chosen) response, and the unpreferred (rejected) response. This structure forms the direct input for training the reward model.

Here's a simplified example of how a single data point might be represented in Python:

preference_data_point = {
    "prompt": "Explain the concept of gradient descent in simple terms.",
    "chosen_response": (
        "Imagine you're blindfolded on a hill and want to get to the bottom. "
        "Gradient descent is like taking small steps in the steepest downhill "
        "direction you can feel with your feet. You keep taking steps until "
        "you can't go further down."
    ),
    "rejected_response": (
        "Gradient descent uses derivatives to find the minimum of a function. "
        "It calculates the gradient and updates parameters."
    ),
    "labeler_id": "annotator_123",
    "timestamp": "2023-10-27T10:30:00Z",
    # Optional fields
    "reasoning": (
        "Chosen response uses a helpful analogy, making it simpler."
    ),
    "model_version": "sft_model_v1.2"
}

# Often stored in formats like JSON Lines or loaded into libraries
# like Hugging Face Datasets
# Example using Hugging Face datasets structure
# dataset = Dataset.from_dict({
#     "prompt": ["prompt1", "prompt2", ...],
#     "chosen": ["chosen1", "chosen2", ...],
#     "rejected": ["rejected1", "rejected2", ...]
# })

Ensuring Data Quality and Scale

The performance of the RLHF process heavily depends on the quality and quantity of the preference data.

Labeler Training and Consistency: Labelers need thorough training on the guidelines and criteria. Regular calibration sessions and monitoring inter-annotator agreement (IAA) are important to ensure consistency. Disagreements should be adjudicated, and guidelines refined if necessary.
Scale: Training effective reward models typically requires a substantial amount of preference data, often ranging from tens of thousands to hundreds of thousands of labeled pairs, depending on the complexity of the target behavior and the diversity of prompts.
Prompt Diversity: As mentioned earlier, the prompts must cover a wide range of topics and styles to ensure the resulting reward model generalizes well. Overfitting the RM to a narrow set of prompts can lead to poor performance during the RL fine-tuning phase.

Example distribution of prompt sources used for collecting preference data.

Challenges in Preference Data Collection

Collecting human preference data is a significant undertaking with several inherent challenges:

Cost and Time: Human annotation is expensive and time-consuming, especially at the scale required for large models.
Subjectivity: Even with clear guidelines, human preferences can be subjective, leading to disagreements among labelers. Capturing a consensus or understanding the distribution of preferences is non-trivial.
Labeler Bias: Labelers might bring their own implicit biases, which could inadvertently be encoded into the reward model if not carefully managed through diverse labeler recruitment and bias training.
Task Complexity: Evaluating complex outputs (e.g., long-form text, code generation, multi-turn dialogue) can be cognitively demanding for labelers, potentially impacting quality and throughput.
Guideline Evolution: As model capabilities change or new issues emerge, the labeling guidelines often need refinement, requiring retraining and potentially relabeling of some data.

Despite these challenges, collecting high-quality preference data is a cornerstone of the RLHF process. This dataset directly shapes the reward signal used to steer the language model towards behaviors aligned with human values. The next step involves using this collected data to train the Reward Model itself.