Skip to main content

Hallucination

The hallucination metric determines whether your LLM generates factually correct information by comparing the actual_output to the provided context.

info

If you're looking to evaluate hallucination for a RAG system, please refer to the faithfulness metric instead.

Required Arguments​

To use the HallucinationMetric, you'll have to provide the following arguments when creating an LLMTestCase:

  • input
  • actual_output
  • context
note

Remember, input and actual_output are mandatory arguments to an LLMTestCase and so are always required even if not used for evaluation.

Example​

from deepeval import evaluate
from deepeval.metrics import HallucinationMetric
from deepeval.test_case import LLMTestCase

# Replace this with the actual documents that you are passing as input to your LLM.
context=["A man with blond-hair, and a brown shirt drinking out of a public water fountain."]

# Replace this with the actual output from your LLM application
actual_output="A blond drinking water in public."

test_case = LLMTestCase(
    input="What was the blond doing?",
    actual_output=actual_output,
    context=context
)
metric = HallucinationMetric(threshold=0.5)

metric.measure(test_case)
print(metric.score)
print(metric.reason)

# or evaluate test cases in bulk
evaluate([test_case], [metric])

There are five optional parameters when creating a HallucinationMetric:

  • [Optional] threshold: a float representing the maximum passing threshold, defaulted to 0.5.
  • [Optional] model: a string specifying which of OpenAI's GPT models to use, OR any custom LLM model of type DeepEvalBaseLLM. Defaulted to 'gpt-4o'.
  • [Optional] include_reason: a boolean which when set to True, will include a reason for its evaluation score. Defaulted to True.
  • [Optional] strict_mode: a boolean which when set to True, enforces a binary metric score: 0 for perfection, 1 otherwise. It also overrides the current threshold and sets it to 0. Defaulted to False.
  • [Optional] async_mode: a boolean which when set to True, enables concurrent execution within the measure() method. Defaulted to True.

How Is It Calculated?​

The HallucinationMetric score is calculated according to the following equation:

Hallucination=Number of Contradicted ContextsTotal Number of Contexts\text{Hallucination} = \frac{\text{Number of Contradicted Contexts}}{\text{Total Number of Contexts}}

The HallucinationMetric uses an LLM to determine, for each context in contexts, whether there are any contradictions to the actual_output.

info

Although extremely similar to the FaithfulnessMetric, the HallucinationMetric is calculated differently since it uses contexts as the source of truth instead. Since contexts is the ideal segment of your knowledge base relevant to a specific input, the degree of hallucination can be measured by the degree of which the contexts is disagreed upon.