# SRM Checker: Sample Ratio Mismatch Test | George Peric
URL: https://www.cro-designer.com/tools/srm-checker
Last updated: 2026-10-26

> Free sample ratio mismatch (SRM) checker: enter visitors per variant and the split you set to see if your A/B test traffic split is broken. Chi-square test.

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Free SRM checker

## Is your A/B test split broken?

Enter the visitors each variant got and the split you set. A chi-square test tells you if the difference is normal chance or a sample ratio mismatch that makes the test unreliable.

### Your split

Use visitors (or users) per variant from your testing tool, for the whole test.

Number of variants

Variant A: visitors

Expected share (%)

Variant B: visitors

Expected share (%)

Variant C: visitors

Expected share (%)

Variant D: visitors

Expected share (%)

Verdict

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Observed split

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Expected split

–

Chi-square

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p-value

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Get help running your tests

Free and private: everything is calculated in your browser. Need someone to plan, design and read your tests? See my A/B testing services.

### What sample ratio mismatch means

A sample ratio mismatch (SRM) means your variants got a different share of traffic than you set, by more than chance can explain. If you set 50/50 and got 52/48 on a large sample, something in the setup is biased: a redirect, a bot filter, a tracking bug or a variant that loads slower. Those problems usually also bias the result, so a test with SRM can’t be trusted.

The checker uses Pearson’s chi-square goodness-of-fit test and flags SRM when the p-value is below 0.01, a common, conservative threshold that avoids false alarms.

### What to do if you find SRM

Don’t read the result. Find the cause first: check redirects, bot filtering, when users are assigned, and whether one variant fails to fire tracking. I walk through the causes and a worked example in sample ratio mismatch: what it is and how to check.

FAQ

### Frequently asked questions

What p-value indicates SRM?

Most teams use p < 0.01, or stricter, as the alarm. A small imbalance is normal; a p-value this low means the split is very unlikely to be chance.

Can I check SRM during the test?

Yes, and you should, early and often. Catching SRM in the first days saves weeks of a broken test.

Does SRM matter if the split is only slightly off?

On a large sample, even a small imbalance can be a real problem. The chi-square test accounts for sample size, so trust the p-value, not your eye.

Should I use visitors or users?

Use the unit your test assigns: usually users or visitors, not page views or sessions.

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### Other A/B testing calculators

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#### A/B test sample size calculator

How many visitors and how many days your test needs before you can trust the result.

#### A/B test significance calculator

Is your winner real or just noise? Get the p-value, the lift and its confidence interval.

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