Variance
The spread of repeated observations around their average.
What Variance does not show
A single result cannot reveal variance, and a low average can coexist with unstable outliers.
Where Variance comes up on this site
- Core Web Vitals without CrUX: field data for small sitesWe measured the spread of a single page's lab score across runs in one Lighthouse run cannot support a finding ; the variance was large enough that one run should never be reported as a site's performance.
- Lighthouse scores vary between runs: two runs, 225% apartTwo readings are not a distribution, so 225% is not "the" variance of Lighthouse.
Terms this definition uses
Related terms in Measurement and evaluation
How a measurement earns trust: evaluation sets, error rates, calibration, reproducibility, and the biases that make a number look better than it is.
AI answer reading · name confusion · hedged answer · Benchmark · Evaluation set · Golden dataset · Ground truth · Human evaluation · LLM-as-judge · Pairwise evaluation · Pointwise evaluation · Rubric · Inter-rater agreement · Accuracy · F1 score · False positive rate · False negative · True positive · True negative · Calibration · Confidence score · Coverage · Abstention · Reproducibility · Repeatability · Confidence interval · Sample size · Sampling bias · Selection bias · Survivorship bias · Data leakage · Test contamination
Questions about Variance
What is Variance?
The spread of repeated observations around their average.
What does Variance not show or guarantee?
A single result cannot reveal variance, and a low average can coexist with unstable outliers.
Which area of the glossary does Variance belong to?
Measurement and evaluation: How a measurement earns trust: evaluation sets, error rates, calibration, reproducibility, and the biases that make a number look better than it is.
← Repeatability · Confidence interval →
See it in the full glossary · 772 terms across 19 areas.
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