Ground truth
Definition
The agreed correct answer for a given input, written down by someone who actually knows. Without ground truth, you can't score anything — you can only vibe-check.
Ground truth is the agreed correct answer for a given input, written down by someone who actually knows. It's the concrete point where domain expertise enters the system instead of staying in someone's head. For some tasks ground truth is a single string; for others it's a rubric, a schema, or a list of required and forbidden elements. In every case, it's the thing an eval is measured against.
Why it matters
No ground truth, no score. Every eval — deterministic or LLM-as-judge — is scored against a ground truth definition. Without it, 'accuracy' is a made-up number and every iteration decision is an opinion.
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