A/B Testing Core Web Vital Calculator FAQ
Answers to common questions about comparing LCP, INP, CLS, and combined Core Web Vitals results in A/B tests.
Use these questions and answers to understand the calculator inputs, percentage changes, combined index, and the limits of using performance metrics to assess an A/B test.
General calculator questions
Basic information about the comparison and its outputs.
What does the A/B Testing Core Web Vital Calculator compare?
It compares control and variant values for LCP, INP, and CLS, then estimates changes for each metric and for a combined equal-weight index.
Which version is better when the overall improvement is positive?
A positive overall improvement means the variant has a lower combined index than the control, which favors the variant on this performance measure.
What does a negative improvement percentage mean?
It means the variant's combined index or individual metric is higher than the control's, indicating a regression for that measure.
Does the calculator score every page experience factor?
No. It covers LCP, INP, and CLS only. It does not include all technical, usability, accessibility, or business factors.
Inputs and measurement
Questions about obtaining comparable metric values.
Should control and variant data come from the same source?
Yes. Use the same data type and collection approach for both versions so the comparison is meaningful.
Can I compare field data and lab data?
They should not be mixed in one comparison. Field data reflects real-user conditions, while lab data is best used for controlled diagnostics.
Why does traffic mix matter?
Metrics can vary by device, network, geography, browser, page template, and user behavior. A different mix can change the reported values without a code change.
What units should I enter?
Enter LCP in seconds, INP in milliseconds, and CLS as its numeric score without converting it to a percentage.
Formula and thresholds
Questions about how the calculations work.
How is LCP improvement calculated?
It is calculated as control LCP minus variant LCP, divided by control LCP, multiplied by 100.
How is the combined CWV index calculated?
The calculator divides LCP by 2.5, INP by 200, and CLS by 0.1, then averages the three ratios for each experience.
Why is the combined index lower when performance is better?
Each index component is a normalized version of a metric where lower values are better. Lower component values produce a lower average.
Are 2.5 seconds, 200 ms, and 0.1 official pass/fail rules in this calculator?
They are reference thresholds used to normalize the three metrics in the calculation. The calculator's index itself is an educational comparison measure.
Accuracy and experiment decisions
How to use the result appropriately.
Does a better Core Web Vitals result prove that a test won?
No. It does not show statistical significance or the effect on conversion, revenue, retention, or other experiment outcomes.
Can a small index difference be noise?
Yes. Small differences may reflect normal variation, especially with limited, unrepresentative, or changing traffic.
What if one Core Web Vital improves and another gets worse?
Review the separate metric changes. The combined index summarizes direction but does not replace metric-by-metric assessment.
Can I use this calculator for performance debugging?
It can help summarize before-and-after or control-versus-variant values, but diagnostic work usually needs metric-level investigation and comparable test conditions.
What Core Web Vitals does this calculator compare?
It compares Largest Contentful Paint, Interaction to Next Paint, and Cumulative Layout Shift for a control and variant.
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