Formula
(Guardrail metric in test - Guardrail metric in control) / Guardrail metric in control x 100
How to measure
Compare the guardrail metric's value in the test group against the control group during an experiment, expressed as a percentage difference.
Why track it
Monitoring guardrail deviation catches situations where a metric win is actually a hidden loss elsewhere, like retention dropping while signups rise.
Example
A signup flow change lifts conversion 12% but also shows a 4% guardrail deviation in 7-day retention, prompting the team to hold off shipping it broadly.