AI Search Metric
Citation diversity index
The number of distinct domains an LLM cites across a prompt set, normalised by prompt count. Falling diversity in a category signals consolidation, and a closing window for new entrants.
What it is
The citation diversity index is the number of distinct domains an LLM cites across a prompt set, normalised by the prompt count. It describes how concentrated or spread out the sourcing is within a category.
Why it matters
Falling diversity in a category signals consolidation around a few trusted sources and a closing window for new entrants, so it is an early warning about how hard future entry will be.
How it works
Practitioners count unique cited domains per prompt set over time and watch the trend, then act faster to establish source authority while the field is still broad.
When it applies
Monitor it when assessing how contestable a category still is or timing an entry into a new topic area.
Examples
- A category where answers once cited a dozen domains now leaning on three
- A new brand entering a topic while diversity is still high and citations are spread
- A strategist using a sustained drop in diversity to justify investing early
How it is measured
- Distinct cited domains divided by prompt count
- Trend in diversity index over successive runs
- Concentration of citations in the top few domains
- Diversity index compared across related categories
Related terms in AI Search Metric
- AI-referred trafficSessions referred from ChatGPT, Claude, Gemini, Perplexity, Copilot, and other LLM surfaces. Often under-counted in standard analytics because referrers are stripped or labelled as direct.
- Answer attribution rateThe proportion of AI answers that explicitly link back to source pages versus those that summarise without attribution. Attribution rate varies materially by platform and query type.
- Bounce-back rate (AI surface)The rate at which users return to the SERP or refine the prompt after seeing an AI Overview. Google's internal signal that an Overview failed, high bounce-back is associated with Overviews being silently removed for that query cluster.
- Brand mention rateThe frequency at which an LLM mentions a brand by name in answers across a prompt set, with or without citation. Brand mentions without citation are an early indicator of model-internalised brand awareness.
- Citation depthHow early in an AI-generated answer a source is cited. Earlier citations correlate with higher click-through from AI surfaces and stronger user trust signals.
- Citation shareThe proportion of times a domain or page is cited inside AI-generated answers (ChatGPT, Claude, Gemini, Perplexity, AI Overviews) across a tracked prompt set. The AI-era equivalent of share-of-voice and the metric most predictive of branded discovery in 2026.