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Podcast - Episode 105

Is word error rate just a vanity metric?

About the show

Hosted by Nikola Mrkšić, Co-founder and CEO of PolyAI, the Deep Learning with PolyAI podcast is the window into AI for CX leaders. We cut through hype in customer experience, support, and contact center AI — helping decision-makers understand what really matters.

Summary

Word error rate is the number most of the voice AI industry can't stop quoting. It might also be the one buyers should trust least.

In this episode, Nikola Mrkšić and PolyAI agent design and engineering lead Oliver Shoulson dig into what a month of analysis across 100,000+ production call turns actually showed about how well voice agents understand people.

Their main finding: word error rate is buried in noise. Most mistranscriptions are harmless. A dropped filler word, a missed article, an inflection that's wrong in a language with heavy morphology — none of it changes what the customer meant. A smaller set are critical entity errors, like a single wrong digit in a phone number, and those are the ones that quietly break a call. When Oliver's team pulled the harmless errors out on their own, they explained less than 1% of the variation in conversation quality. Nikola calls them empty calories.

The bigger surprise: task success stayed almost perfectly flat as meaning-changing errors piled up. LLMs recover from mishearings far better than expected — callers pay in a little friction, not failure. The conversation also covers where context-aware ASR still falls short, and a blunt take on full-duplex interfaces like GPT live that are built to be talked over.

For anyone evaluating voice AI, here's the takeaway: stop buying on benchmark word error rate. Start measuring task success and conversation quality on your own calls.

Key takeaways

  • Not all words are created equal: Classifying mistranscriptions by business impact — benign, semantic, and critical — shows that the benign ones (filler words, articles, formatting) explain under 1% of the variation in conversation quality. Judging an ASR system on raw word error rate means buying on noise.
  • LLMs recover from errors better than you think: Task success stayed almost perfectly flat even as meaning-changing errors accumulated — callers pay in extra clarification turns, not in failed calls. The real cliff in conversation quality only appears at three or more meaning-changing errors, a cohort of just tens of calls in the whole dataset.
  • Production reality beats vendor benchmarks: The headline production word error rate was 6.4% for English and 8.8% for Spanish (6.7% weighted) — nowhere near the 2–3% vendors quote from clean datasets, once phone low-pass filters, VoIP packet loss, and callers driving down the highway enter the picture.
  • Semantic and critical errors behave in opposite ways: Semantic errors get punished on quality (−0.55) because they force clarification, but barely move handoffs. Critical entity errors — a wrong digit in a phone number — hit quality half as hard (−0.27) because they escalate to a human quickly and appropriately. The future is context-aware ASR that sees the conversation, not blind transcription.