## SRM - sample ratio mismatch

Sample ratio mismatch (SRM) occurs when the observed allocation of **unique** users between test groups differs from the expected allocation or split of the test. Refer to the [Statsig blog post on sample ratio mismatch](https://www.statsig.com/blog/sample-ratio-mismatch) for background.

SRM is a signal that there may be unknown bias in the test. This is a significant problem because, without a clear diagnosis of the imbalance, it is difficult to know how much the bias affects your results.

## SRM checks

Statsig runs SRM checks on all experiments and feature gates as part of Health Checks (refer to [Health Checks](/experiments/monitor)). Statsig uses a Chi-squared test to identify whether the split of users between groups indicates a sample ratio mismatch.

{% figure %}
![SRM health check results interface](/images/experiments/srm-checks-health.png)
{% /figure %}

Statsig automatically analyzes data by common dimensions logged by the Statsig SDK to identify potential drivers of SRM. These include sdk\_type, sdk\_version, reason, is\_bot, browser\_name, browser\_version, os, os\_version, and region to identify potential causes.

{% figure %}
![SRM dimension analysis breakdown](/images/experiments/srm-checks-dimensions.png)
{% /figure %}