Field data describes users; a lab test diagnoses conditions
Use lab tests to diagnose performance and field evidence to understand measured visitor experience. A fast test on one machine does not prove that visitors on slower devices experience the same result.
A laboratory run observes one controlled environment. Field measurements summarize eligible real-user experience under their collection scope. Device mix, network conditions and page interactions can make the two disagree without either measurement being fabricated.
A worked example
A hypothetical desktop lab test loads a lightweight cached page quickly, while mobile users encounter a large uncached hero image and a busy consent interface. The lab result does not invalidate their experience. A targeted mobile trace can help identify the cause, but it remains a diagnostic sample rather than a replacement for field evidence.
How to apply it
- Record whether the result is URL-level, origin-level or a single lab run.
- Preserve device, period and data-availability labels in the report.
- Use traces to diagnose the problem and comparable field evidence to monitor real-user experience.
What the result can establish
When field data is unavailable, say so instead of borrowing a neighboring page’s value. In SharkBlade, distinguish supported field KPI evidence from the raw crawl score. An improved lab run is a verified test result, not proof that all visitors or rankings improved.
Reference: Google: Core Web Vitals.
Supporting documentation
Google: Core Web Vitalsweb.dev: optimizing LCPweb.dev: optimizing INPweb.dev: optimizing CLSGoogle: technical requirements