04.04 · Walkthrough
Field vs Lab: CrUX, RUM and Lighthouse
Explain why a Lighthouse score and a CrUX report disagree, and choose which one to trust for a given decision.
Lighthouse is controlled lab evidence, CrUX is aggregated Chrome field evidence, and RUM is your own production telemetry. They disagree because they measure different users, devices, caches, networks and page states. Trust Lighthouse for debugging, CrUX for external real-user Core Web Vitals context, and RUM for segmented operational decisions.
What this lesson answers
- why Lighthouse and CrUX scores disagree
- when to trust CrUX over Lighthouse
- what is RUM for Core Web Vitals
Notes
Lighthouse is a lab test: it loads a page in a controlled environment with chosen device, network, browser settings, and auditing rules. CrUX is field data: aggregated measurements from real Chrome users who visited real pages under their actual devices, networks, locations, cache states, and interaction patterns. RUM is your own field data, collected by instrumentation you add to your site, usually with more detail and faster feedback than CrUX but only for traffic you measure correctly.
The mental model is unit test versus production telemetry. Lighthouse is excellent for reproducing a…
Common questions
- Why does Lighthouse fail while CrUX looks fine?
- Lighthouse runs a controlled test with specific throttling, browser settings and cache conditions. CrUX aggregates visits from real Chrome users, who may have faster devices, warmer caches or different networks. A poor lab result can reveal a real implementation risk even when the current user population is mostly avoiding that worst case.
- Is CrUX more important than Lighthouse for SEO decisions?
- For search-related page experience and Core Web Vitals status, CrUX is the closer match because it reflects real Chrome users over time. Lighthouse is still valuable, but mainly for finding and verifying fixes on a page. Use CrUX to judge field outcome, then use Lighthouse to investigate causes.
- When should I use RUM instead of CrUX?
- Use RUM when you need detail CrUX does not expose: logged-in sessions, experiments, routes, releases, countries, device classes or business flows. CrUX is external and comparable, but limited in segmentation. RUM gives faster, more specific production feedback if your instrumentation is accurate and covers the traffic you care about.
Short definition: what is Field vs Lab: CrUX, RUM and Lighthouse?