# Side-by-side eval

**Source:** https://promtable.com/glossary/side-by-side-eval

> Side-by-side eval presents two model or prompt outputs to a rater (human or LLM judge) for direct comparison — "which one is better?" — instead of grading each on an absolute scale.

---
Side-by-side eval presents two model or prompt outputs to a rater (human or LLM judge) for direct comparison — "which one is better?" — instead of grading each on an absolute scale.

Side-by-side evals (also called pairwise comparison) often produce more reliable judgments than absolute scoring because raters anchor on the comparison rather than an arbitrary 1-5 scale. Used heavily in 2026 when promoting a new prompt or model: take the golden set, run both, present pairs to LLM judges + a human sample, pick the winner per pair, aggregate. Tools: Braintrust, LangSmith pairwise evals, custom Streamlit dashboards. Best practice: randomise order to avoid position bias, run multiple LLM judges to reduce single-judge bias.

## When to use

- Promoting a new prompt or model variant.
- Choosing between close alternatives where absolute scores hard to calibrate.

## Common mistakes

- Not randomising order — judges have position bias.
- Single LLM judge — calibrate with a jury or human spot-check.

## Related terms

- [evals](https://promtable.com/glossary/evals)
- [ab-testing-prompts](https://promtable.com/glossary/ab-testing-prompts)
- [llm-jury](https://promtable.com/glossary/llm-jury)
- [vibe-eval](https://promtable.com/glossary/vibe-eval)

*Last updated: 2026-06-01*
---

Original page: https://promtable.com/glossary/side-by-side-eval
Maintained by Promtable (https://promtable.com). Content: CC BY 4.0. Cite as "Promtable — https://promtable.com/glossary/side-by-side-eval".
Contact: info@vibecodingturkey.com.