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Prompt-based Hallucination Metric#

The prompt-based hallucination metric uses a strong Large Language Model (LLM) judge, such as OpenAI's GPT-4, to detect hallucination. The metric can be computed in various ways depending on the prompt given to the LLM. Depending on the prompt technique used, the metric can be a boolean value, where True indicates hallucination, or a numeric value ranging from 0 to 1, where 0 indicates no hallucination and 1 indicates definite hallucination.

Implementation Details#

The main prompting techniques we will focus on in this guide are:

  1. Chain-of-Thought Prompt - This prompting technique asks the judging model to answer with a yes or no if the generated response contains hallucination. When the judge detects hallucination, the Chain-of-Thought (CoT) prompting technique is used to improve the model's reasoning capabilities.
  2. Self-Consistency Prompt - This technique involves prompting the judging model multiple times with the same prompt asking it to detect hallucinations and aggregating the outputs to obtain a score, known as a hallucination score.

In practice, we recommend using a combination of the self-consistency prompting technique and the CoT technique.

In the following section, we will compute the prompt-based hallucination metric using these two prompting techniques on example responses. We will use GPT-4 as the judging model, as it is currently the most effective at detecting hallucinations.

Example#

Given the following set of ground truths and inferences, we will compute the hallucination metric using the two prompting techniques.

Ground Truth Inference
The duck crossed the road The duck did not cross the road
The duck crossed the road The animal crossed the road
The duck crossed the road The duck may not be the one who crossed the road

To make API requests to OpenAI GPT models, you need to install the OpenAI Python library and set up your API key. If you don't have a secret key yet, you can create one on OpenAI's API key page.

pip install openai
export OPENAI_API_KEY=`your-api-key-here`

After setting up an API key, let's prompt the judging model of your choice for each pair of ground truth and inference.

Chain-of-Thought Prompt#

Using the CoT prompting technique, we can prompt the judging model to evaluate hallucination in a boolean format and ask for its reasoning. Here is an example of the CoT prompt technique:

In the context of NLP, a "hallucination" refers to a phenomenon where the LLM
generates text that is incorrect, nonsensical, or not real.

Given two texts, where the first one is a perfect answer, and the second one
is a generated answer, if the generated answer is considered a "hallucination",
return "yes". Otherwise, simply return "no". If yes, on the next line, explain
in about ten words why there is a hallucination by thinking step by step.

Tips on Prompt for Question Answering Workflow

By providing more context along with the expected and generated answers, you can achieve more accurate evaluation. We recommend adding the question to the answer in the prompt.

If you are working on a closed-domain or retrieval-augmented generation (RAG) type dataset, it is recommended to provide the reference or retrieved text along with the question and answers to provide more context to detect hallucinations.

How to Prompt gpt-4
import os
from openai import OpenAI

openai = OpenAI(api_key=os.environ["OPENAI_API_KEY"])

PROMPT = """
In the context of NLP, a "hallucination" refers to a phenomenon where the LLM generates text that is incorrect, \
nonsensical, or not real.

Given two texts, where the first one is a perfect answer, and the second one is a generated answer, if the
generated answer is considered a "hallucination", return "yes". Otherwise, simply return "no".
If yes, on the next line, explain in about ten words why there is a hallucination by thinking step by step.
"""

response = openai.chat.completions.create(
    model="gpt-4-1106-preview",
    messages=[
        {
            "role": "user",
            "content": PROMPT,
        },
        {
            "role": "assistant",
            "content": "Certainly! Please provide me with the texts for evaluation.",
        },
        {
            "role": "user",
            "content": f"Perfect Answer: {ground_truth}" f"\n\nGenerated Answer: {inference}",
        },
    ],
    temperature=0.5,
    max_tokens=50,
    top_p=1,
    frequency_penalty=0,
    presence_penalty=0,
)

response = str(response.choices[0].message.content)
Ground Truth Inference GPT-4 Judge Response Reasoning
The duck crossed the road The duck did not cross the road yes The generated answer contradicts the perfect answer directly.
The duck crossed the road The animal crossed the road no -
The duck crossed the road The duck may not be the one who crossed the road no -

Based on GPT-4's response, we can identify any potential hallucination in each generated answer.

Self-Consistency Prompt#

In the previous section, we learned about the CoT prompting technique. In this section, let's repeat the same prompt five times and measure the hallucination score. If you are unsure how many times to repeat, starting with five is a good option. The hallucination score is calculated as the rate of the number of times the judging model detected hallucination to the total number of instances:

\[ \text{hallucination score} = \frac{\text{number of hallucinating instances}}{\text{total number of instances}} \]
Ground Truth Inference GPT-4 Judge Responses Hallucination Score
The duck crossed the road The duck did not cross the road [yes, yes, yes, yes, yes] 1.0
The duck crossed the road The animal crossed the road [no, no, no, no, no] 0.0
The duck crossed the road The duck may not be the one who crossed the road [no, no, yes, yes, no] 0.4

Based on GPT-4's response, the inference in the first example pair is definitely hallucinating, and the one in the second pair is factually consistent. However, the inference in the last pair seems to be neutral or inconclusive, as the judging model predicted yes twice out of the five times it was prompted.

Limitations and Advantages#

  1. Cost - Running a large model entails significant expenses. The cost of operating an API model such as GPT-4 is determined by the number of tokens used. However, if you employ your own model as a judging model, the payment might not be calculated based on the token count; nevertheless, there will be additional costs for hardware, computation and maintenance. It is essential to remember that monetary expenses are not the sole consideration. You should also take into account the inference time of the judging model.

  2. Privacy and Security - To achieve desirable results, you need access to a sufficiently performant LLM. However, using GPT-4 or similar models through an API can hold privacy and security concerns when datasets are meant to be kept private.

While using LLMs to detect hallucinations has limitations, it also offers advantages:

  1. Improved Accuracy - LLM prompt-based evaluation is one of the state-of-the-art techniques being used for detecting hallucinations in LLMs.

  2. Explainability - By using a performant LLM as a judging model, it can provide explanations for flagging a response as a hallucination. These explanations can help us understand the reasoning behind the hallucination detection of a given prompt.