Product metrics and goals
Guardrail metric
Also called Counter metric, Health metric
A metric you agree not to make worse while chasing a goal, so improving one number doesn't quietly damage another.
How it's worked out
A threshold on a metric that must hold: 'unsubscribes stay below 0.5%'
Any target can be gamed. More notifications raise engagement and unsubscribes; bigger discounts raise orders and lower margin. A guardrail catches that.
In experiments, a test that wins on its main metric but breaks a guardrail doesn't ship.
In practice
A team trying to raise weekly sessions watches notification opt-outs and app store ratings as guardrails.
Common mistake
Listing so many guardrails that nothing can ever ship. Pick the few that would genuinely hurt.
Where it comes from
Common in online experimentation, and described in Trustworthy Online Controlled Experiments (2020) by Ron Kohavi, Diane Tang and Ya Xu.