analytics-dictionary / term / experimentation

Sample ratio mismatch (SRM)

A sample ratio mismatch is when the actual split of users between variants differs from the intended split.

Written by Reflective Data experts · updated Oct 5, 2026

If you planned 50/50 and observe 52/48 with a large sample, something is wrong: bugs in assignment, bot traffic, redirects or tracking. Results from tests with an SRM cannot be trusted.

Automated SRM checks are one of the most valuable quality controls in experimentation.

A/B test Data quality Statistical significance

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