Hallucination Evaluation

Vectara uses the Hughes Hallucination Evaluation Model (HHEM) to assess the likelihood of AI-generated summary being factually consistent based on search results. This calibrated score can range from 0.0 to 1.0. A higher score indicates a higher confidence that the summary is factually consistent, while a lower score indicates possible hallucinations.

For example, a score of 0.95 suggests a 95% likelihood that the summary is free of hallucinations and would align with the original content. A lower score of 0.40 indicates a 40% chance the summary is free of hallucinations, meaning it's more likely to contain one or more factual inaccuracies, however minor. We suggest starting with a setting of 0.5 as an initial guideline.

note

The FCS measures only the factual accuracy of a generated summary based on
search results. Each search result's independent score is based on the query
settings used for that query (e.g. which embedding model is used, whether
lambda and/or reranking is used). These scores are different.

Factual Consistency Score language support

The Factual Consistency Score supports English, German, French, Portuguese, Spanish, Arabic, Chinese-Simplified, Korean, Russian (rus), Japanese (jpn), and Hindi (hin). Set the response_language parameter to eng, deu, fra, spa, por, ara, kor, zho, rus, jpn, or hin.

HHEM 2.3 also introduces architectural improvements that significantly reduce latency and computational costs, enhancing overall model performance.

Enable the Factual Consistency Score

In your summarization request, set the enable_factual_consistency_score field to true. The Factual Consistency Score returns a calibrated value in the factual_consistency_score field of the summary message. The score field contains the value between 0.0 and 1.0.

Enable the Factual Consistency Score

"generation": {
    "generation_preset_name": "vectara-summary-ext-24-05-med-omni",
    "max_used_search_results": 5,
    "enable_factual_consistency_score": true
  }
<CodePanel snippets={[{language: "bash", code: `In the following example, the summary shows a \`factual_consistency_score\` of
\`0.98\`, which is 98%.`}]} title="Code Example" layout="stacked" />json showLineNumbers title="Example Factual Consistency Score"
{
  "summary": "According to the novel 'The Hitchhiker's Guide to the Galaxy' by Douglas 
    Adams, the answer to the ultimate question of life, the universe, and everything is 42.",
  "summary_language": "en",
  "factual_consistency_score": 0.98,
  "search_results": [
    // ...
  ]
}